Aiokatcp

- Make things work on MacOS again.
- CI: Update certifi to a newer version.
- Add a type annotation to prevent an internal error in old versions of mypy.

- Workaround to prevent old versions of mypy (0.780) from throwing an internal
error.

- Add :class:`.DeviceStatus` enum for discrete device-status sensors.
- Add :class:`.TimeSyncUpdater` to assist in writing sensors that monitor time
synchronisation.

- Make PEP 604 union syntax work in handler annotations.
- Speed up message parsing, particularly for messages with thousands of arguments.

- Add :class:`.SimpleAggregateSensor` class to simplify common use cases for
aggregate sensors.
- Improve error reporting when requests are made with too few arguments,
particularly when the handler is wrapped in a decorator.
- Allow handler arguments to be annotated as :class:`Optional[T]`. It's not
possible to provide a ``None`` value on the wire, but this allows the
default value to be ``None`` while complying with mypy's strict mode.
- Update :program:`katcpcmd` and the examples to use :func:`asyncio.run`
instead of manually running the event loop. This eliminates some deprecation
warnings.
- Update pre-commit hook to point to flake8's new Github URL.
- Test on Python 3.11.

- Replace ``None`` with ``''`` for a default sensor description. If no description
is given, the ``?sensor-list`` request would fail. This is now fixed.
- Add :class:`.AggregateSensor` functionality, an abstract class of sensors which
depend on the readings of others for their values.