Enable mypy lint in CI

[codetheweb]

I recently enabled most of our lints in CI: #2046.

I didn't enable mypy in that PR because it looked like it required a few manual changes to pass the lint. We need to enable this lint by updating .github/workflows/chroma-lint.yml and making the appropriate changes to pass the lint (test locally with pre-commit run --all-files mypy).

[ibratoev]

@codetheweb You can assign this to me. There are a ton of errors to fix. I will start opening PRs in batches to make it easier to review.

[codetheweb]

awesome, thank you so much!