Aiokatcp

- 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.

- Fix a resource leak that prevented full cleanup of client state on the
server after the client disconnected (although the garbage collector would
have cleaned it up eventually).

- Use bulk sensor sampling in :class:`.SensorWatcher` when available.

- Add support for :doc:`server/service_tasks`.

- Increment server protocol version to 5.1.
- Implement bulk sensor sampling feature of katcp 5.1.
- Bump minimum Python version to 3.7, and run test suite against 3.10.
- Change type annotation of :attr:`.Server.server` from
:class:`asyncio.AbstractServer` to the more specific
:class:`asyncio.Server`.
- Add :attr:`.Server.sockets` to simplify querying the sockets of a server.
- Lots of internal code modernisation (f-strings, PEP 526 type annotations,
isort, and so on).