[ambivalence]

You can see how this was done for Fabric, PyPA/Warehouse, and pytest.

General guidelines:

1. One commit with only the automatic formatting. Afterwards you'll be able to skip over it easily with `git hyper-blame` or `git blame $BLACK_REV^ -- $FILE`.

2. Avoid leaving open pull requests. If you do, after landing the blackening commit, blacken all pull requests, too. They shouldn't conflict then.

3. Set up enforcement with pre-commit or CI (you can run `black --check` on Travis or similar).

4. Don't forget the repo badge ;-)