mrhlpr: Add type hints and appease mypy

This does not cover the entirety of mrhlpr's codebase, and does not even attempt to start typing mrtest. The types were generated with MonkeyType, which seems to have worked well to get a starting point for typing. However, it still required significant manual adjustment and as such time investment due to some of the type hints being wrong (presumably since MonkeyType runs off of runtime information and I didn't manage to trigger every possible code path), but primarily due to the codebase needing other fixes to make mypy happy.

Overall, I think MonkeyType saved me enough time to be worthwhile using for future type expansion, but the time investment required is still significant.