what is the right environment for type checking

[d-v-b]

We declare a bunch of dependencies in our mypy pre-commit check. pre-commit resolves these to create a completely separate environment just for mypy.

I think It would probably be better to re-use one of our developer environments, and even better if the exact developer environment was reproducible, e.g. by including a lockfile in version control.

[d-v-b]

see #3972 for one direction. that PR re-uses the developer environment for running mypy.