Replace Optional["T"] with "T | None"

Replace Optional["T"] with "T | None"

In Python 3.10 strings don't support the | operator, so ruff doesn't
attempt to fix these. Quote the entire type specification to avoid the
typing.Optional import.

Alternatives I considered:
- leaving it as is (only use of Optional in the code base)
- using `from future import __annotations__` (replacing one import with
  another one)