Python packaging + installability

[akx]

This PR reorganizes the repo (just unpeels the interoperability directory) and adds a basic pyproject.toml so the repository can be used as a regular Python package as well, e.g.

pip install git+https://github.com/eclipse/paho.mqtt.testing

and the mqtt (namespace) package becomes available.

It also adds a basic GitHub Actions workflow to build the package.

Note

This will of course necessarily break previous uses that have e.g. added paho.mqtt.testing/interoperability to sys.path by hand, but hopefully those repositories will have pinned their paho.mqtt.testing submodule to a version they know to be working.

This is related to the renovation efforts over at paho.mqtt.python:

• Modernize build (to Hatch) paho.mqtt.python#774
• Convert homespun tests to conventional tests paho.mqtt.python#780
• (Kind of) fix mqttv5 tests paho.mqtt.python#781