Enforce mypy in pre-commit

[gabegma]

Description:

mypy was never enforced in the pre-commit because, at a certain time, we couldn't put the errors to 0 easily. This is no longer the case, and I don't think we should be able to commit with mypy errors.

Checklist:

You should check all boxes before the PR is ready. If a box does not apply, check it to acknowledge it.

• ISSUE NUMBER. You linked the issue number (Ex: Resolve #XXX).
• PRE-COMMIT. You ran pre-commit on all commits, or else, you
  ran pre-commit run --all-files at the end.
• USER CHANGES. The changes are added to CHANGELOG.md and the documentation, if they impact
  our users.
• DEV CHANGES.
  • Update the documentation if this PR changes how to develop/launch on the app.
  • Update the README files and our wiki for any big design decisions, if relevant.
  • Add unit tests, docstrings, typing and comments for complex sections.

[JosephMarinier]

Oh! Very good catch! Thank you!
I just have a small comment to clean it up a bit.