Establish Python formatting guidelines

[benpankow]

It would be good to set up some sort of linting/formatting requirement before the codebase grows. I'm partial to black, which is part of the PSF. It has plugin support for most major editors and can be set up with CI to check for proper formatting at PR time.

flake8 checks code compliance with the PEP 8 Python style guide. We could run this after black to catch any other linting errors. The downside of solely using flake8 is that it won't fix these issues for you - black will happily format your code and fix most problems.

Of course, introducing style requirements does raise the bar to commit a bit. If we do implement style guidelines, we should ensure it's as easy as possible to install and run the needed tools.