Add type checking in CI and fixup annotations

[PeterJCLaw]

This adds type checking in CI with mypy, a basic configuration for mypy and fixes the errors that are found.

The configuration is very nearly the default and mostly doesn't enable additional checks. The checks which are enabled are mostly to guide future usage in the right direction.

The fixes here are intended to be as minimal as possible and are derived from code inspection. I've run the tests, but don't have an actual deployment to test this with manually.