Consider testing `.pyi` files with `mypy.stubtest()` in CI

`mypy` provides [tools for stub testing](https://mypy.readthedocs.io/en/stable/stubtest.html). We should investigate using this as part of CI to make sure we keep the `.pyx` and `.pyi` synced. That said, I am not 100% sure it will play nicely with Cython. Needs investigation.