Use pre-commit to run mypy and flake8
Use pre-commit to run mypy and flake8

Developers can use `pre-commit install` to configure pre-commit hooks to
run mypy any flake8 as pre-commit git hooks.

We can also leverage pre-commit.ci to run these for PRs. It's super
fast, and we don't need to duplicate any definitions.

pdav/khal,geier/khal,pimutils/khal