Update to require Python 3.8

[mwchase]

Services and libraries of interest:

• ReadTheDocs (I could ask about getting on the testing branch, once I'm ready to merge)
• Travis
• Appveyor
• tcod
• pony apparently (needed for mutmut)

[mwchase]

This update will do excellent things for the quality, because it'll allow for a lot of code deletion and improved annotations, as well as additional functionality for Functions and Methods.

[mwchase]

See libtcod/python-tcod@8c467e2

[mwchase]

It'd be great if homebrew's pyenv update could get a successful Jenkins run, please and thank you.