RFC: drop Python 3.8 support? 3.9? 3.10?

[DanAlbert]

#299 was uncontentious. Anyone still using Python 3.8? 3.9 itself isn't very exciting (mostly some syntax improvements for type annotations), but there's a fair amount of good stuff in 3.10 and 3.11, and we need to drop 3.8 and 3.9 before we can use those.

If no one is stuck on old python versions we may as well unconstrain ourselves, but if anyone's still reliant on these versions there's no rush.

(In any case we should wait a while since the 3.7 drop hasn't actually gone out yet)