CLN: Condense PR style checklist into one script
A script is easier to execute and manage as we manage our style-checking tools.

Started with `flake8`, `black`, and `isort`, as those are the main ones for Python-related changes (for reference, we didn't even have `isort` in the checklist beforehand).

If we're happy with the structure, we can always add more OR modify checks if we want going forward while keeping it easy for folks to check their PR's.