Test Type Annotations

[rmorshea]

Current Situation

Currently many public facing type annotations are not exercised. One way to do this might be to run MyPy against our test suite. However this makes it more time consuming to write tests that people already don't want to write. I'd worry this would deter contributors from testing their code thoroughly.

Proposed Actions

pytest-mypy-testing provides a simple framework for making assertions about the result of reveal_type. This seems like the best way to check type annotations without incurring extra costs while writing normal tests.