Remove all trace of flake8-mypy (#1867)

flake8-mypy is long dead and shouldn't be used, see
https://github.com/ambv/flake8-mypy. We appear to use pre-commit to run
mypy now anyway.

I ran `pipenv uninstall flake8-mypy`, which seems to have made several
changes to Pipfile.lock. Let me know if there's a better way to do this.

Co-authored-by: hauntsaninja <>