Clean up using ruff better

We're using ruff with default rules. This is overall quite ok, but we could consider throwing in some N (naming) and D rules to pytest-ruff in CI, since these are helpful with keeping up the code quality. At the moment, we are breaking some naming rules with AA and BB in the names, but that's justified, I think. Still, running ruff check . --extend-select Nshows that we could improve a few variable names, and ruff check . --extend-select D shows that we have undocumented variables.