Add Ruff, a fast Python linter to the project's precommit, and fix the findings

I wanted to try this linter for a while, and I actually found that what it signalled made sense. I spent some time fixing the warnings and now we can set this up as a pre-commit to make sure we don't have any issues creeping up.

I don't really think the linter found anything that was a real bug (thanks to our test suite), but it definitely caught things that were leftover after a refactoring, like a variable that is now unused still laying around. I think it can improve the quality of the code.