ci: Add mypy

[LecrisUT]

None of the code is type-hinted, so there would be some effort to go through for this one. But at least here is the tooling if you want to steadily tackle it

[github-actions[bot]]

Thank you for making this pull request.

Did you know? You can try it on Binder: or .

Also, the version of Jupytext developed in this PR can be installed with pip:

pip install git+https://github.com/LecrisUT/jupytext.git@ci/mypy

(this requires nodejs, see more at Developing Jupytext)