Enable negative narrowing of Union TypeVar upper bounds

[brianschubert]

Fixes #15235

Before

from typing import TypeVar

class A:
    pass
class B:
    b: int

T = TypeVar("T", bound=A | B)


def foo(x: T) -> T:
    if isinstance(x, A):
        reveal_type(x)      # N: Revealed type is "__main__.A"
    else:
        reveal_type(x)      # N: Revealed type is "T`-1"
        x.b                 # E: Item "A" of the upper bound "A | B" of type variable "T" has no attribute "b"
    return x

After

from typing import TypeVar

class A:
    pass
class B:
    b: int

T = TypeVar("T", bound=A | B)


def foo(x: T) -> T:
    if isinstance(x, A):
        reveal_type(x)      # N: Revealed type is "__main__.A"
    else:
        reveal_type(x)      # N: Revealed type is "T`-1"
        x.b                 # ok! Upper bound of T narrowed to B
    return x

[brianschubert]

I believe this is consistent with the way pyright applies negative narrowing to TypeVars with union upper bounds: Pyright playground

[github-actions]

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

artigraph (https://github.com/artigraph/artigraph)
- src/arti/internal/type_hints.py:177: error: Argument 2 to "_check_issubclass" has incompatible type "T"; expected "type"  [arg-type]

[hauntsaninja]

Thanks!