Run Mypy inside pytest/Nox instead of pre-commit

[MicaelJarniac]

Mypy quite often needs a complete set of requirements installed in order to function properly.

pre-commit creates a fresh environment for each hook and basically doesn't install anything other than the hook itself.

We can provide additional dependencies for pre-commit to install along with the hook, but that'd mean duplicating the requirements and having to maintain them both up-to-date.

A pre-commit hook can be run from the external environment, instead of its isolated one, but that'd then make it dependant on a properly configured environment, which won't always be the case, and that'd limit where pre-commit can be run from.

Another option would be to have the hook install the full set of requirements, but that'd add a considerable slowdown.

Since running pytest already requires a fully working environment, that could be a nice place to have Mypy run.

Perhaps a subset of Mypy features can still be called from pre-commit.

https://jaredkhan.com/blog/mypy-pre-commit
pre-commit/pre-commit#880 (comment)
https://github.com/dbader/pytest-mypy

[MicaelJarniac]

Maybe use Nox for running Mypy?

Related to #7.