[BE] Apply pep585 and turn on flake8 rule so no regressions

[Lucaskabela]

Since python>=3.10 is defined in the pyproject.toml, we can enable pep585 (in python >=3.9)

Why?

1. Fewer imports making code simpler
2. Standardization - prior to this change, we could have List and list intermixed in the codebase
3. Future proof for direction of python type system

Test

Ran

 python -m flake8 --pep585-activation=always .

Explicitly

[fegin]

Nice BE task, but you have to rebase after #2386 is landed.

[Lucaskabela]

Hi @fegin - should be ready for review :)