Mypy checks pass in pre-commit but fail when run in Github

[bsowell]

Mypy sometimes succeeds when run in pre-commit, but fails with legitimate type issues when run in isolation or in Github (example job).

This seems to be a somewhat common issue (e.g. see pypa/pip#9502), though I haven't had success with any of the proposed workarounds. There are some more ideas here, though they aren't entirely satisfactory. Part of the issue seems to be how pre-commit works by running in a separate virtualenv.

In the meantime, running poetry run mypy --install-types . in the root of the project appears to surface the correct errors.