Deprecate Python 2.7 support, make Python 3.8 the minimum

[goodmami]

Python 2 is soon obsolete and Python 3.4 is already out of support. Python 3.5 is still supported but probably nearing its end. Python 3.6 is the default Python 3 version for the latest Ubuntu LTS (18.04) so let's go with that as the minimum version we officially support.

As of Nov 2023, Python 3.8 is the minimum, and 3.12 is the latest.

[goodmami]

To make this more concrete so we know when to close it, here's a small list of things to do:

• Remove Python 2 workarounds (search for __future__, NameError, ImportError, "Python 2" and fix as necessary)
• Maybe undo some changes from 3b6313b
• Ensure the README and other documents say Python 3.6+ is required