Pretty code

[kstrauser]

I wanted to make the code prettier and more editor-friend. This PR:

• Uses Black to format the code
• Uses isort to sort the imports at the top of each module
• Updates the type hints so that mypy passes cleanly
• Removes almost all wildcard imports (which helps editors better know where names are defined)
• Found a test (see f92b887) that had been nested inside another, and therefore hadn't run, in about 8 years!
• Cleaned up a lot of pylint and flake8 warnings and errors, but tried not to go too far down that rabbit hole

I think the end result looks nice and modern. 🙂

This builds on #620 and shouldn't be merged until that one is, and then after rebasing.