Use mypy in pre-commit

[nicoddemus]

Since we are using type annotations, I think we should enable mypy in the pre-commit configuration.

We might also consider configuring mypy to demand type annotations for all parameters and return types in pytest_rich.py, while also enabling full type checking even in non-annotated functions under tests/.

[joshuadavidthomas]

I have a branch for this, but will probably need a few cleanup PRs before merging it because there are quite a few type errors to work through.

A couple in the RichTerminalReporter, but embarrisingly for me, most of them are in the custom traceback class I wrote. 😳

[nicoddemus]

Ahh that is absolutely normal, if no tool is checking it, it is easy to introduce erros!