…iding-black-version-in-github-action

Since we're moving to a composite based action, quite a few changes
were made. 1) Support was added for all OSes (Windows was painful).
2) Isolation from the rest of the workflow had to be done manually
with a virtual environment.

Other noteworthy changes:

- Rewrote basically all of the logic and put it in a Python script
  for easy testing (not doing it here tho cause I'm lazy and I can't
  think of a reasonable way of testing it).
- Renamed `black_version` to `version` to better fit the existing
  input naming scheme.
- Added support for log groups, this makes our action's output a
  bit more fancy (I may or may have not added some debug output too).

Reflect compatability and gotchas.

…thub-action