Support and test Python 3.10

[felix-hilden]

🥳 Let's also add PyPI classifiers for 3.6-10

[felix-hilden]

We'll most likely have to wait until Sphinx 2.3 is released with 3.10 support. I'm guessing the errors in CI are because of it.

[felix-hilden]

It wasn't because of it. Honestly I have no idea.

[felix-hilden]

Now I know exactly what is going on:

• Pytest cannot import requests, as reported in python_files=*.py breaks with Python 3.10 pytest-dev/pytest#9174
• This is a bug in CPython: https://bugs.python.org/issue45419
• It was fixed on 10.10. in [3.10] bpo-45419: Fix interfaces on DegenerateFiles.Path python/cpython#28844
• No Python 3.10 releases have been made since the initial release on 4.10. (what a missed opportunity it was to release on the 4th...)

So we need to wait for 3.10.1 to be released, which is actually in 3 days on Monday😄

[felix-hilden]

It was released, and now we have to wait until it is added to the GH Action.

[felix-hilden]

Okay, well. In the most recent test runs we do install Python 3.10.1, but the problem still persists.