diff --git a/python-backend/musicmouse/__main__.py b/python-backend/musicmouse/__main__.py
index 3dc8b79..06f5b93 100644
--- a/python-backend/musicmouse/__main__.py
+++ b/python-backend/musicmouse/__main__.py
@@ -19,7 +19,7 @@ import logging
 import sys
 from collections.abc import Awaitable, Callable, Coroutine
 from pathlib import Path
-from typing import Any
+from typing import Any, TypeVar
 
 import httpx2
 
@@ -324,7 +324,10 @@ def _build_app(
     return app
 
 
-def _service_of_type[T: Service](services: list[Service], kind: type[T]) -> T | None:
+T = TypeVar("T", bound=Service)
+
+
+def _service_of_type(services: list[Service], kind: type[T]) -> T | None:
     return next((s for s in services if isinstance(s, kind)), None)
 
 
diff --git a/python-backend/musicmouse/bus.py b/python-backend/musicmouse/bus.py
index 6d8b9d9..ddd691c 100644
--- a/python-backend/musicmouse/bus.py
+++ b/python-backend/musicmouse/bus.py
@@ -16,7 +16,7 @@ import contextlib
 import inspect
 import logging
 from collections.abc import Callable, Coroutine
-from typing import Any
+from typing import Any, TypeAlias, TypeVar
 
 from musicmouse.events import Event
 
@@ -24,8 +24,12 @@ _log = logging.getLogger(__name__)
 
 __all__ = ["EventBus", "Handler", "Unsubscribe"]
 
-type Handler[E: Event] = Callable[[E], Coroutine[Any, Any, None] | None]
-type Unsubscribe = Callable[[], None]
+#: An alias carrying a TypeVar is generic on its own, so ``Handler[SomeEvent]`` still
+#: parameterises it the way the PEP 695 form did.
+E = TypeVar("E", bound=Event)
+
+Handler: TypeAlias = Callable[[E], Coroutine[Any, Any, None] | None]
+Unsubscribe: TypeAlias = Callable[[], None]
 
 
 class EventBus:
@@ -63,7 +67,7 @@ class EventBus:
 
     # --------------------------------------------------------------- subscription
 
-    def subscribe[E: Event](self, event_type: type[E], handler: Handler[E]) -> Unsubscribe:
+    def subscribe(self, event_type: type[E], handler: Handler[E]) -> Unsubscribe:
         """Register ``handler`` for ``event_type`` and any subclass of it."""
         self._handlers.setdefault(event_type, []).append(handler)
         self._resolved.clear()
diff --git a/python-backend/musicmouse/config.py b/python-backend/musicmouse/config.py
index 4812a50..c97c729 100644
--- a/python-backend/musicmouse/config.py
+++ b/python-backend/musicmouse/config.py
@@ -9,7 +9,7 @@ from __future__ import annotations
 
 import logging
 from pathlib import Path
-from typing import Annotated, Any, Final, Literal, Self
+from typing import Annotated, Any, Final, Literal, Self, TypeAlias
 
 from pydantic import (
     BaseModel,
@@ -51,7 +51,7 @@ __all__ = [
 ]
 
 #: Number keys on the IR remote, as lircd's ``BTN_0``..``BTN_9`` map to them.
-type Digit = Literal["0", "1", "2", "3", "4", "5", "6", "7", "8", "9"]
+Digit: TypeAlias = Literal["0", "1", "2", "3", "4", "5", "6", "7", "8", "9"]
 
 DEFAULT_AUDIO_EXTENSIONS = (".mp3", ".ogg", ".oga", ".opus", ".flac", ".wav", ".m4a", ".aac")
 
diff --git a/python-backend/musicmouse/devices/wire.py b/python-backend/musicmouse/devices/wire.py
index 867b4c3..d0ebe30 100644
--- a/python-backend/musicmouse/devices/wire.py
+++ b/python-backend/musicmouse/devices/wire.py
@@ -15,7 +15,7 @@ from __future__ import annotations
 import struct
 from dataclasses import dataclass, replace
 from enum import IntEnum
-from typing import override
+from typing import TypeAlias
 
 from musicmouse.effects import (
     EffectAlexaSwipeConfig,
@@ -162,7 +162,7 @@ class FirmwareLog:
     text: str
 
 
-type Decoded = InputEvent | FirmwareLog
+Decoded: TypeAlias = InputEvent | FirmwareLog
 
 
 # ------------------------------------------------------------------------- encoding
@@ -367,12 +367,11 @@ class SetButtonBrightness:
     button: Button
     brightness: float
 
-    @override
     def __repr__(self) -> str:
         return f"{self.button.slug} backlight <- {self.brightness:.2f}"
 
 
-type HostCommand = SetEffect | SetButtonBrightness
+HostCommand: TypeAlias = SetEffect | SetButtonBrightness
 
 _ID_TO_EFFECT: dict[int, tuple[LedZone, type[LedEffect]]] = {
     message: (zone, effect_cls)
diff --git a/python-backend/musicmouse/events.py b/python-backend/musicmouse/events.py
index 099670c..a09eb8d 100644
--- a/python-backend/musicmouse/events.py
+++ b/python-backend/musicmouse/events.py
@@ -15,7 +15,7 @@ from __future__ import annotations
 
 import time
 from dataclasses import dataclass, field
-from typing import Literal
+from typing import Literal, TypeAlias
 
 from musicmouse.effects import LedEffect
 from musicmouse.hardware import Button, ButtonAction, LedZone, RotaryDirection, TouchButton
@@ -53,7 +53,7 @@ __all__ = [
     "VolumeChanged",
 ]
 
-type EventSource = Literal["device", "player", "mqtt", "web", "lirc", "simulator", "system"]
+EventSource: TypeAlias = Literal["device", "player", "mqtt", "web", "lirc", "simulator", "system"]
 
 
 @dataclass(frozen=True, slots=True, kw_only=True)
diff --git a/python-backend/musicmouse/library/models.py b/python-backend/musicmouse/library/models.py
index 97e1c5c..70b9063 100644
--- a/python-backend/musicmouse/library/models.py
+++ b/python-backend/musicmouse/library/models.py
@@ -5,6 +5,7 @@ from __future__ import annotations
 import hashlib
 from dataclasses import dataclass
 from pathlib import Path
+from typing import TypeAlias
 
 from musicmouse.library.analysis import TrackAnalysis
 from musicmouse.library.sections import AlbumKind
@@ -15,7 +16,7 @@ __all__ = ["Album", "AlbumColors", "LibraryTrack", "album_id", "track_key"]
 #: Primary, secondary and accent as ``"#rrggbb"`` - the format
 #: :func:`musicmouse.color.parse_color` already accepts, so the LED side needs no new
 #: parsing and the frontend gets CSS colours for free.
-type AlbumColors = tuple[str, str, str]
+AlbumColors: TypeAlias = tuple[str, str, str]
 
 
 def album_id(root: Path, folder: Path) -> str:
diff --git a/python-backend/musicmouse/library/sections.py b/python-backend/musicmouse/library/sections.py
index c8fc41e..7990fe8 100644
--- a/python-backend/musicmouse/library/sections.py
+++ b/python-backend/musicmouse/library/sections.py
@@ -8,15 +8,15 @@ quirks are not understood yet.
 from __future__ import annotations
 
 from dataclasses import dataclass
-from typing import Final, Literal
+from typing import Final, Literal, TypeAlias
 
 __all__ = ["SECTIONS", "AlbumKind", "ArtistSource", "Section", "TitleSource", "TrackOrder"]
 
-type AlbumKind = Literal["music", "book"]
-type TrackOrder = Literal["filename", "newest_first"]
-type TitleSource = Literal["tags", "folder"]
-type ArtistSource = Literal["tags", "folder"]
-type AlbumUnit = Literal["folder", "episode"]
+AlbumKind: TypeAlias = Literal["music", "book"]
+TrackOrder: TypeAlias = Literal["filename", "newest_first"]
+TitleSource: TypeAlias = Literal["tags", "folder"]
+ArtistSource: TypeAlias = Literal["tags", "folder"]
+AlbumUnit: TypeAlias = Literal["folder", "episode"]
 
 
 @dataclass(frozen=True, slots=True)
diff --git a/python-backend/musicmouse/reactions/registry.py b/python-backend/musicmouse/reactions/registry.py
index b38d1e7..bd5f3f3 100644
--- a/python-backend/musicmouse/reactions/registry.py
+++ b/python-backend/musicmouse/reactions/registry.py
@@ -8,7 +8,7 @@ from __future__ import annotations
 
 import logging
 from collections.abc import Callable, Coroutine
-from typing import TYPE_CHECKING, Any
+from typing import TYPE_CHECKING, Any, TypeAlias, TypeVar
 
 from musicmouse.bus import EventBus
 from musicmouse.events import Event
@@ -20,12 +20,14 @@ _log = logging.getLogger(__name__)
 
 __all__ = ["Reaction", "on", "register_all", "registered"]
 
-type Reaction[E: Event] = Callable[[E, "App"], Coroutine[Any, Any, None] | None]
+E = TypeVar("E", bound=Event)
+
+Reaction: TypeAlias = Callable[[E, "App"], Coroutine[Any, Any, None] | None]
 
 _REGISTRY: list[tuple[type[Event], Reaction[Any]]] = []
 
 
-def on[E: Event](event_type: type[E]) -> Callable[[Reaction[E]], Reaction[E]]:
+def on(event_type: type[E]) -> Callable[[Reaction[E]], Reaction[E]]:
     """Register a reaction for ``event_type`` (and any subclass of it)."""
 
     def decorator(reaction: Reaction[E]) -> Reaction[E]:
@@ -46,7 +48,7 @@ def register_all(bus: EventBus, app: App) -> None:
     _log.debug("Registered %d reactions", len(_REGISTRY))
 
 
-def _bind[E: Event](reaction: Reaction[E], app: App) -> Callable[[E], Any]:
+def _bind(reaction: Reaction[E], app: App) -> Callable[[E], Any]:
     def handler(event: E) -> Any:
         return reaction(event, app)
 
diff --git a/python-backend/musicmouse/simulator/driver.py b/python-backend/musicmouse/simulator/driver.py
index 29e0dbe..dba3a3a 100644
--- a/python-backend/musicmouse/simulator/driver.py
+++ b/python-backend/musicmouse/simulator/driver.py
@@ -17,6 +17,7 @@ from __future__ import annotations
 
 import logging
 from dataclasses import dataclass
+from typing import TypeVar
 
 from musicmouse.app import App
 from musicmouse.clock import Clock, FakeClock
@@ -329,9 +330,10 @@ def _int(text: str) -> int:
         raise ScriptError(f"{text!r} is not a whole number") from None
 
 
-def _enum_by_name[T: (Button, ButtonAction, TouchButton)](
-    enum: type[T], name: str, what: str
-) -> T:
+_EnumT = TypeVar("_EnumT", Button, ButtonAction, TouchButton)
+
+
+def _enum_by_name(enum: type[_EnumT], name: str, what: str) -> _EnumT:
     try:
         return enum[name.upper()]
     except KeyError:
diff --git a/python-backend/musicmouse/tippen/curriculum.py b/python-backend/musicmouse/tippen/curriculum.py
index 09bb0b1..5e5403c 100644
--- a/python-backend/musicmouse/tippen/curriculum.py
+++ b/python-backend/musicmouse/tippen/curriculum.py
@@ -15,7 +15,7 @@ from __future__ import annotations
 import logging
 from dataclasses import dataclass
 from pathlib import Path
-from typing import Final, Literal
+from typing import Final, Literal, TypeAlias
 
 from pydantic import BaseModel, ConfigDict, ValidationError, model_validator
 from ruamel.yaml import YAML
@@ -37,9 +37,9 @@ __all__ = [
     "world_reward",
 ]
 
-type LessonKind = Literal["letters", "fragments", "words", "sentences"]
-type ModeId = Literal["dive", "bubbles", "feed", "race"]
-type CreatureId = Literal["clownfish", "octopus", "seahorse", "turtle", "pearlmussel"]
+LessonKind: TypeAlias = Literal["letters", "fragments", "words", "sentences"]
+ModeId: TypeAlias = Literal["dive", "bubbles", "feed", "race"]
+CreatureId: TypeAlias = Literal["clownfish", "octopus", "seahorse", "turtle", "pearlmussel"]
 
 #: In world order - see ``tippen/src/lib/aquarium.ts``, the one place this list is
 #: allowed to grow, since each id names a drawing under ``public/aquarium/``.
diff --git a/python-backend/pyproject.toml b/python-backend/pyproject.toml
index e3e6495..30cc8fc 100644
--- a/python-backend/pyproject.toml
+++ b/python-backend/pyproject.toml
@@ -2,7 +2,11 @@
 name = "musicmouse"
 version = "2.0.0"
 description = "Host backend for the MusicMouse RFID music player"
-requires-python = ">=3.13"
+# Raspberry Pi OS (Bookworm) ships Python 3.11, and its system interpreter is what the
+# device runs. Staying on it means apt and piwheels supply prebuilt armhf wheels for
+# the native dependencies (pydantic-core, Pillow), instead of needing a separate
+# toolchain to fetch a newer interpreter and compile them on the Pi.
+requires-python = ">=3.11"
 dependencies = [
     "aiomqtt>=2.0",
     "fastapi>=0.115",
@@ -50,7 +54,7 @@ filterwarnings = ["error"]
 
 [tool.ruff]
 line-length = 100
-target-version = "py313"
+target-version = "py311"
 # Course material and scratch work, not part of the backend. See notebooks/README.md.
 extend-exclude = ["notebooks"]
 
@@ -63,7 +67,7 @@ select = ["ARG", "B", "C4", "E", "F", "I", "N", "PTH", "RUF", "SIM", "UP", "W"]
 "musicmouse/services/mqtt/entity.py" = ["ARG002", "B027"]
 
 [tool.mypy]
-python_version = "3.13"
+python_version = "3.11"
 strict = true
 files = ["musicmouse"]
 warn_unreachable = true
diff --git a/python-backend/tests/test_lirc.py b/python-backend/tests/test_lirc.py
index 4349e36..1f9574f 100644
--- a/python-backend/tests/test_lirc.py
+++ b/python-backend/tests/test_lirc.py
@@ -70,6 +70,7 @@ class ScriptedLircd:
     def __init__(self) -> None:
         self._writer: asyncio.StreamWriter | None = None
         self._connected = asyncio.Event()
+        self._closing = asyncio.Event()
         self.connection_count = 0
 
     async def _handle(self, _reader: asyncio.StreamReader, writer: asyncio.StreamWriter) -> None:
@@ -77,7 +78,14 @@ class ScriptedLircd:
         self.connection_count += 1
         self._connected.set()
         with contextlib.suppress(asyncio.CancelledError):
-            await asyncio.Event().wait()  # held open until the test drops it
+            await self._closing.wait()  # held open until the test drops it
+        writer.close()
+        with contextlib.suppress(OSError, asyncio.CancelledError):
+            await writer.wait_closed()
+
+    def shutdown(self) -> None:
+        """Let go of every connection still being held open, so the server can close."""
+        self._closing.set()
 
     async def wait_connected(self) -> None:
         await self._connected.wait()
@@ -108,8 +116,10 @@ async def lircd() -> AsyncIterator[tuple[ScriptedLircd, asyncio.base_events.Serv
     finally:
         server.close()
         # The scripted connection handler holds its connection open forever (it does
-        # not know the test is done), so `wait_closed()` alone would hang here.
-        server.close_clients()
+        # not know the test is done), so `wait_closed()` alone would hang here. 3.13
+        # has Server.close_clients() for exactly this; ask the handlers to let go
+        # instead, because the device runs the Pi's Python 3.11.
+        station.shutdown()
         await server.wait_closed()
 
 
diff --git a/python-backend/tests/test_mouse.py b/python-backend/tests/test_mouse.py
index 16a0ecf..37dbad7 100644
--- a/python-backend/tests/test_mouse.py
+++ b/python-backend/tests/test_mouse.py
@@ -2,6 +2,7 @@ from __future__ import annotations
 
 import logging
 from collections.abc import AsyncIterator
+from typing import TypeVar
 
 import pytest
 
@@ -58,7 +59,10 @@ def seen(bus: EventBus) -> list[Event]:
     return events
 
 
-def only[T: Event](events: list[Event], event_type: type[T]) -> list[T]:
+T = TypeVar("T", bound=Event)
+
+
+def only(events: list[Event], event_type: type[T]) -> list[T]:
     return [e for e in events if isinstance(e, event_type)]
 
 
diff --git a/python-backend/tests/test_player.py b/python-backend/tests/test_player.py
index 7987a30..f8a653f 100644
--- a/python-backend/tests/test_player.py
+++ b/python-backend/tests/test_player.py
@@ -10,6 +10,7 @@ from __future__ import annotations
 
 from collections.abc import AsyncIterator
 from pathlib import Path
+from typing import TypeVar
 
 import pytest
 
@@ -55,7 +56,10 @@ def seen(bus: EventBus) -> list[Event]:
     return events
 
 
-def only[T: Event](events: list[Event], event_type: type[T]) -> list[T]:
+T = TypeVar("T", bound=Event)
+
+
+def only(events: list[Event], event_type: type[T]) -> list[T]:
     return [e for e in events if isinstance(e, event_type)]
 
 
diff --git a/python-backend/tests/test_reactions.py b/python-backend/tests/test_reactions.py
index 83e0bd5..51967a8 100644
--- a/python-backend/tests/test_reactions.py
+++ b/python-backend/tests/test_reactions.py
@@ -8,6 +8,7 @@ from __future__ import annotations
 
 from collections.abc import AsyncIterator
 from pathlib import Path
+from typing import TypeVar
 
 import pytest
 
@@ -49,7 +50,10 @@ def seen(sim: Simulation) -> list[Event]:
     return events
 
 
-def only[T: Event](events: list[Event], event_type: type[T]) -> list[T]:
+T = TypeVar("T", bound=Event)
+
+
+def only(events: list[Event], event_type: type[T]) -> list[T]:
     return [e for e in events if isinstance(e, event_type)]
 
 
diff --git a/python-backend/tests/test_web.py b/python-backend/tests/test_web.py
index 28cee3e..90ad1fb 100644
--- a/python-backend/tests/test_web.py
+++ b/python-backend/tests/test_web.py
@@ -11,6 +11,7 @@ import contextlib
 import json
 from collections.abc import AsyncIterator
 from pathlib import Path
+from typing import TypeAlias
 
 import httpx2
 import pytest
@@ -27,7 +28,7 @@ from tests.websocket_harness import WebSocketSession, websocket_connect
 TRACK_SECONDS = 10.0
 
 #: Shorthand: every test takes the same client type.
-type Client = httpx2.AsyncClient
+Client: TypeAlias = httpx2.AsyncClient
 
 
 @pytest.fixture