Fix isinstance with type aliases to PEP 604 unions

[hauntsaninja]

Fixes #12155, fixes #11673, seems pretty commonly reported issue

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅

[JukkaL]

What about old-style unions? They work at runtime:

>>> from typing import Union
>>> isinstance(1, Union[int, str])
True

It's also okay to only support PEP 604 unions for now.

[JelleZijlstra]

Note those only work with isinstance() in 3.10+.

[hauntsaninja]

I'll take a look at supporting old-style unions, but in another PR. It doesn't seem to be nearly as popular a feature request and it would require monkeypatching typeshed or doing more isinstance special casing: python/typeshed#12137

[hauntsaninja]

This also fixes #13810

[ngnpope]

@hauntsaninja Unfortunately it seems this fix doesn't extend to the case where the type alias is defined in another module:

Given a.py:

from typing import TypeAlias

IntOrStr: TypeAlias = int | str

assert isinstance(1, int | str)
assert isinstance(1, IntOrStr)

And b.py:

from a import IntOrStr

assert isinstance(1, int | str)
assert isinstance(1, IntOrStr)

I get the following output:

$ mypy --version
mypy 1.11.0 (compiled: yes)
$ mypy --strict a.py
Success: no issues found in 1 source file
$ mypy --strict b.py
b.py:4: error: Parameterized generics cannot be used with class or instance checks  [misc]
b.py:4: error: Argument 2 to "isinstance" has incompatible type "<typing special form>"; expected "_ClassInfo"  [arg-type]
Found 2 errors in 1 file (checked 1 source file)

The issue seems to be that uses_pep604_syntax is False on this line.