General fixes for next release

[Yobmod]

Some fixes for next release:

• fix classifiers list in setup.py - tested with testpypi
• Update types of cmd.handle_process_output()
• Apply workaround for protocol bug that appeared in 3.9.7 and 3.10.0.rc1 (at runtime they are now ABCs). This might already be fixed for 3.10.0-rc2, so can then be reverted once 3.10.0 released bpo-44806: Fix issue with typing.Protocol __init__ method generation python/cpython#27541 TypeError: Protocols cannot be instantiated #1332
• Update Sphinx to 4.1.12 and fix warnings - still not 3.10 compatible tho. 3.1.20: sphinx 4.x warnings #1323
• Update version and changelog

The CI for 3.10.0 failed on doc building for beta4 and rc1 (with different errors), but newest sphinx not working with 3.10, so waiting for that: sphinx-doc/sphinx#9505)

I've uploaded to testpypi, then install from there with 3.7.0 and got no import errors.
Also added tests for earliest available minor versions for each major one (e.g 3.7.5, 3.8.0, 3.9.0)

So ready for release, I guess!