Fix bug in constraints solver regarding ParamSpec upper bounds

[AlexWaygood]

Description

Fixes #12930.

Currently, mypy erroneously emits two errors on this snippet:

from typing_extensions import ParamSpec
from typing import Callable, TypeVar, Iterator, Iterable, Tuple

P = ParamSpec("P")
T = TypeVar("T")

def enumerate(x: Iterable[T]) -> Iterator[Tuple[int, T]]: ...

def foo(func: Callable[P, T]) -> Callable[P, T]:
    def _inner(*args: P.args, **kwargs: P.kwargs) -> T:
        y = enumerate(args)  # E: Need type annotation for "y"  # E: Argument 1 to "enumerate" has incompatible type "P.args"; expected "Iterable[<nothing>]"
        return func(*args, **kwargs)
    return _inner

This is because of a bug in constraints.py: ParamSpecArgs and ParamSpecKwargs have upper bounds following #12668 (tuple[object, ...] and dict[str, object] respectively), but the constraints solver isn't currently aware of that fact. This PR fixes that bug.

Test Plan

I added a test to one of the existing test cases.

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉