Add github action to check formatting

[jenhagg]

What the code does

First commit defines the action, which installs black and runs the check on only the pyreisejl folder, outputting the diff if it fails. I used this instead of the action provided by black repo since that one doesn't enable specifying the directory (would work either way, just felt this was more explicit).
Second commit is the result of running black locally so the check will pass.

Note: as we add more workflows to CI, this will probably be simplified by calling tox and moving the workflow logic to a tox.ini so we can run the same command locally and in a github action (rather than call several separate things - black, pytest, isort, etc). Wanted to share the idea but this note can be ignored for now.

Time to review

5 mins, probably

[rouille]

The usual simple quote to double quote