update style to more modern python idioms and remove dead imports

[aaraney]

This PR does introduce behavioral changes. Its highly recommended to view each commit individually during review. Ive split things up by the refactoring / stylistic operation that was performed to make this easier to review.

The refactors / stylistic improvements performed are:

chore:

• remove unused imports
• add from __future__ import annotations to relevant files

refactor:

• guard non-runtime types with typing.TYPE_CHECKING
• single member typing.Union

style:

• use __future__ annotations and remove quotes from, now, non-runtime type hints
• drop open's default 'r'
• use f-strings

Note: non-typing types (e.g. list[str]) do not work with pydantic for python<=3.8. See this issue for more information.

[aaraney]

@hellkite500, I think we should be good to merge this now.