mrhlpr: Add type hints and appease mypy (MR 31)

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.

This also introduces the MergeRequestStatus and GitLabOrigin dataclasses
to avoid unnecessary type assertions and improve ease of reading the
code.