no mp when parallel is false, also ruff

[corinwagen]

I added code to avoid using multiprocessing when parallel=False, as I was getting pickling errors. I think this may be OS-dependent; multiprocessing's default process spawn method switches between spawn and fork depending on the operating system used, which is messy. (This also avoids the mp.Process startup time, which can be substantial.)

I also ran the code through the ruff formatter / linter automatically through nvim—no need to merge these, happy to pull out the formatting changes.

The test suite passes locally except for the multiprocessing tests, which are still giving the the aforementioned error:

FAILED tests/test_multiproc.py::test_parallel_run_proc_single_worker - AttributeError: Can't pickle local object 'test_parallel_run_proc_single_worker.<locals>.wrapper'