[securedrop-proxy] black in dev env and CI

[redshiftzero]

In #55 we ran black on the codebase. Let's:

1. Add black as a dev requirement
2. Run it as part of our lint target
3. Run black --check (such that a build fails if black would reformat our code) somewhere in CI before the lint target runs (since if changes are needed make lint would reformat the code) - if you have a better idea here feel free to implement it