Update docs build and test to more recent Python

[rosteen]

Is there a good reason that our docs build and doc tests are still on 3.7, or have they just not been updated in a while? It drives me crazy when I test doc updates in my normal 3.9 env and everything looks good, only to find that they fail in the 3.7 environment up on Github.

CC @eteq @nmearl @keflavich