Standardize and automate python formatting, linting, typechecking, etc

We're growing more and more python in our codebase. We have some facilities for checking it in CI (qv the python3-mypy target); we should get them all in one place, and make sure they are invocable locally too with something like a maint/check-python.

cc @jnewsome @opara

(Assigning to myself, but please feel free to steal this ticket if it seems fun.)