ci: check types with mypy
ci: check types with mypy

This will result in _partial_ type checking of the type annotations by
using mypy. Keep in mind though that mypy is performing _static_
analysis in a _dynamic_ language so it can only partially check for
correctness.

Some other tool(s) will be needed to have more complete type checking at
runtime. E.g. [typeguard].

[typeguard]: https://pypi.org/project/typeguard/

gitpython-developers/gitpython,gitpython-developers/GitPython,gitpython-developers/GitPython,gitpython-developers/gitpython