Support PEP 604 syntax for runtime use cases

[hauntsaninja]

This is a follow up to #9610.

Most of PEP 604 support is available, however we're missing support for isinstance(x, int | str) (and more generally, runtime uses of __or__ on types). This will require a typeshed change, but I'm filing here so that we make sure it works. Even if all it takes is a typeshed change, at the very least we should add a test.

(For people who stumble upon this thread, this is only relevant for Python 3.10 and greater)

[hauntsaninja]

Should also make sure to test base classes: class C(A[int | None]): ... and aliases X = int | str