Rule request: detect redundant type unions

[robsdedude]

Summary

Inspired by #18508

I'd like to see ruff detect (and possibly fix) redundant types in unions. Examples:

import typing

foo1: A | B | A  # becomes A | B
foo2: typing.Union[int, B, float, int, C]  # becomes Union[int, B, float, C]
foo3: typing.Optional[None]  # becomes None

For simplicity, I'd be fine leaving out nested annotations in the first iteration. They could still be added at a later time. E.g.,

import typing

foo: typing.Union[int, None, typing.Optional[int]]  # could become typing.Union[int, None]

[AlexWaygood]

Thanks! We already have this one: https://docs.astral.sh/ruff/rules/duplicate-union-member/#flake8-pyi-pyi

[robsdedude]

Ah, nice! I've not checked it yet but judging by the linked issue that rule does not cover Optional[None]. Would you be interested in a PR to change that?

[AlexWaygood]

Sure, that sounds in the spirit of the rule!

[AlexWaygood]

Might have to be a preview-only change initially though, since it's an expansion in scope