chore: add and update type hints

[jackwotherspoon]

This PR does two things:

1. Adds missing return type hints to unblock ci: update mypy type check #78
2. Updates type hints to use standard collections (list, tuple, dict).
   We can do this now that Python 3.8 was removed

As of Python 3.9 and PEP 585 standard collections can be subscripted in favor
of their typing counterparts which are now deprecated.