Update Python versions

[djmitche]

Partially addresses #484

[djmitche]

This got a little more complicated because nose is abandoned, so I had to rewrite to use pytest. Also pep8 is not a thing anymore.

[djmitche]

This diff is quite large because I switched to black instead of pep8. I'm open to other suggestions! At any rate, it should be reviewable commit-by-commit.

[jwhitlock]

Thanks @djmitche!

I've been using this to mark a commit as "ran a new formatting tool":

https://docs.github.com/en/repositories/working-with-files/using-files/viewing-a-file#ignore-commits-in-the-blame-view