TYP: Add mypy as a pre-commit
This is quite helpful in developing with typing, especially if you plan to update the mypy version.

- [ ] closes #xxxx
- [ ] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

thanks @xhochy 