Aiokatcp

- 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).

- Drop support for Python 3.5, and test on versions up to 3.9.
- Remove explicit ``loop`` arguments.
- Fix a race condition that could cause lost connections to be logged twice.
- Switch testing from nosetests to pytest.
- Switch CI from Travis CI to Github Actions.
- Use a :file:`pyproject.toml` to specify build-time dependencies.
- Upgrade Sphinx used for readthedocs to the latest version.

- Add :meth:`.SensorSet.add_add_callback`, :meth:`SensorSet.remove_add_callback` and
:meth:`SensorSet.remove_remove_callback`.