Fix issues with type aliases and new style unions

[JukkaL]

Fix aliases like this and other aliases involving new-style unions:

A = type[int] | str

Fixes #12392. Fixes #14158.

[ilevkivskyi]

Looks good, but there is one important comment.

[ilevkivskyi]

This should now be handled in various visitors. Three cases come to my mind: First, treetransform.py this is needed so that this error will not re-appear in a test case like this

T = TypeVar("T", int, str)
def foo(x: T) -> T:
    A = type[int] | str
    return x

Second, aststrip.py, be sure that when you switch imported names from types to variables, you do get an error about missing __or__ on update. Third, semanal_typeargs.py (uses MixedTraverserVisitor), since we should not carry malformed instances around (with number of type args), they may cause crashes, add a test just in case with a malformed instance in | alias.

And in general adding in to the basic TraverserVisitor is a good idea. Maybe just grep for def visit_index_expr( and see where we use analyzed.

[JukkaL]

Very good points! I'll fix these.

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

pydantic (https://github.com/samuelcolvin/pydantic)
+ pydantic/networks.py:16: error: Unused "type: ignore" comment