Switch from mypy to red knot

[cthoyt]

Red Knot is the codename for Astral's type checker that will be part of ruff. Assuming it is as fast and high quality as everything else they make, I hope it will be a drop-in replacement for mypy.

Keep up with the churn at https://github.com/astral-sh/ruff/labels/red-knot

Further, this will be reasonable to add into a pre-commit hook even for moderately sized codebases.