Big refactoring: only git, Py 3.6-3.10, no buildout, GHA

[mauritsvanrees]

This does most parts of issue #374. Probably best to review this one commit at a time. Note: I changed the Dockerfile, but did not test it. It might also no longer be needed.

Remaining from that issue, but not for this PR:

• Run black over the whole code. Are you okay with the defaults?
• Get rid of doctests. But this is likely a lot of work. BTW, the doctests may contain old code that pyupgrade did not touch and black will not touch, because it is in a text file.
• Use pytest instead of zope.testrunner and z3c.testsetup.

[reinout]

Wow, I'm going to take "a" look :-)

[reinout]

Black defaults are fine, btw.

[mauritsvanrees]

@reinout I have pushed some minor fixes to address your comments.

[reinout]

Party!