Remove support for Python 3.9

[bluetech]

It will be end-of-life (or on its last breaths) by the time of the next release.

I tried to cover everything, but might have missed something.

The biggest annoyance is the lint which forces explicit strict on zip calls. I was tempted to add the lint to ignores, but it's actually a good lint so I audited all zip calls, hopefully I got it right. It did bring up #13723.

Refs #13719

[Pierre-Sassoulas]

Love the new strict=True ! I reviewed the diff, might take a look at what might be missing later. Adding match cases should be done in another MR, right ?

[bluetech]

Some fixes for CI failures:

• pylib doesn't like PYTHONWARNDEFAULTENCODING=1, changed the tox.ini to avoid it for pylib. This only worked because the CI job ran on py3.9 which I guess didn't have the warning.

• Changed the TerminalWriter zip strict to False, as it failed in (some!) CI jobs. Probably related to highlighting. Pretty suspicious but I couldn't reproduce locally (though didn't try too hard), so for now keeping the existing behavior, but added a comment.

[nicoddemus]

Thanks @bluetech!

[nicoddemus]

Ditto.

[Pierre-Sassoulas]

This one was not removed ?

[bluetech]

• Removed a no longer needed Pypy special case in a test which caused pypy CI (updated 3.9 -> 3.10) to fail

• Adopted @nicoddemus's suggestions, except I kept the README -- I think it's helpful there -- and also a couple in the CONTRIBUTING because I think it's explicitly trying to explain how to run on a specific pyython version.

@Pierre-Sassoulas

Adding match cases should be done in another MR, right ?

Yes, if there are places that can be improved with match/case we can do it separately.

[Pierre-Sassoulas]

Opened #13727 for match cases as a follow-up

[bluetech]

I'll merge this so we can start depending on Python 3.10.