Format the codebase

[AlecRosenbaum]

Related: #12

This change:

• adds a requirements_lint.txt file for linter requirements
• formats code with black
• adds a static-code-analysis job to CircleCI, which enforces code formatting

This is a pretty big diff which murders git history, but the files look much prettier now IMO. Note that the formatting is done with the default black settings. This means there's a line-length of 88 and strings are normalized.

[jkemp101]

Do we want to switch to double quotes for this repo?

[AlecRosenbaum]

I think so. Trying to adhere to a formatting convention explicitly not supported by black is not great IMO. In other projects where quotes aren't enforced due to the single quote convention, I have spent what felt like completely wasted time manually going back through changes and swapping quotes.

[thomasst]

I did not review the specific formatting changes done by black, but enforcing default black settings SGTM.