run mypy during CI / from pre-commit

It's great to have type information, but it's not so great if nothing is verifying it. Let's run mypy during CI, and preferably during pre-commit so that we're aware of the quality of our type annotations.

There will be some challenges to doing this, and so it may not be something to enable by deafult. Still, cookie cutter should help point the way (e.g., by a commented out block in pre-commit-config.yaml) even if we can't enable it by default.

(Though enabling by default means that a library has to be mypy "approved" before its initial release, which would be a good thing if it's feasible)

I am prototyping this in https://github.com/jepler/Jepler_CircuitPython_udecimal/pull/13

Start by reviewing pre-commit-config.yaml and the repository's CI configuration, then compare the prototype in Jepler_CircuitPython_udecimal pull request 13. Determine whether mypy should be enabled by default or documented as an optional block; done means the chosen configuration runs mypy in CI and, where feasible, from pre-commit.

Written by the indexing model from the issue text.