Aiokatcp

- 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`.

- Add `auto_strategy` parameter to :class:`.Sensor` constructor.
- Disconnect clients that aren't keeping up with their asynchronous informs.

- Fix the type annotations to allow :meth:`.Client.add_sensor_watcher` to take a
:class:`.AbstractSensorWatcher` instead of a :class:`.SensorWatcher`.
- Always call sensor watchers in the order they were added.

- Add :meth:`.Server.on_stop`.

- Make :class:`~.SensorSet` more generic and move into :mod:`aiokatcp.sensor`
package. It no longer takes a list of connections; instead, one may register
callbacks to get notification of removals. Note that the constructor
interface has changed in a non-compatible way.
- Add :meth:`.Sensor.Status.valid_value`.
- Add :meth:`.Client.add_inform_callback` and :meth:`.Client.remove_inform_callback`.
- Add support for :ref:`sensor_watcher`.