CI: auto-format code on push

[rland93]

I think it might be useful to enforce code formatting whenever pushes are made.

I use an auto-formatter (python black) and greatly prefer it, because it helps make python syntax more automatic. But it's not just my preference, I think it would help everyone read code across the project since idiosyncratic whitespace styles would be made uniform.

https://black.readthedocs.io/en/stable/integrations/github_actions.html