Move to using black auto-formatting to enforce PEP8.

[MicahGale]

Description

In the OpenMC style guide automated formatting of C++ code is required, and PEP8 compliance is required for python, however no autoformatting is used for python AFAIK. Black is a PEP8 formatting tool for python.

Suggestion for using black:

1. Add black as an optional dependency
2. Update style guide to require using black
3. Reformat all python using black. This would be a large commit
4. Implement a GHA test that checks if any python files need reformatting.

Alternatives

Do nothing, and continue relying on reviewers determining if code is PEP8.

Compatibility

No API changes would be needed. This would add an additional test, and would reformat the whole code base.

[shimwell]

Super keen to see this

Some similar discussions in this related issue #2002

By the way I think Ruff is perhaps also good option to consider

[MicahGale]

Oh I didn't see that issue. I'll close this one.