Add warning for PEP 604 when not in annotation

[wookie184]

Ruff doesn't warn on this:

import typing

y = typing.Union[int, str]
z = typing.Optional[str]

This seems to be as a result of #3215, although I don't really understand the reasoning there, the same change is made if a type is used in an annotation, and as it can still be accessed at runtime it could still cause errors.

For example,

from typing import Union

new_types = (int, float)
def thing(arg: Union[new_types]):
    ...

becomes

new_types = (int, float)
def thing(arg: new_types):
    ...

(really the issue there seems to be invalid use of Union in the first place, afaik the only variables you can use there are type aliases, which that tuple is not)

This is also the case in the other issue linked, #3215, where as far as I can tell the issue happens whether the union is defined in a variable or directly in the annotation.

Anyway, I don't really mind too much about autofixes, but I think it would be nice to be able to have a warning for it regardless.

[charliermarsh]

I think it makes sense to warn on them at least. I'm hesitant to fix since it can introduce subtle bugs, such as that identified in #3215, whereby replacing Union[new_types] with new_types leads to a runtime error (since we can't know that new_types is a tuple there, and not a single type).