Fix checking of match sequence pattern against bounded type variables

[brianschubert]

Fixes #18089

Adds handling for bounded type variables when checking match sequence patterns.

Previously, this crashed. Now, this correctly narrows the upper bound of the type variable:

from typing import TypeVar, Sequence

T = TypeVar("T", bound=Sequence[int | str])

def accept_seq_int(x: Sequence[int]): pass

def f(x: T) -> None:
    match x:
        case [1, 2]:
            accept_seq_int(x)  # ok: upper bound narrowed to Sequence[int]
        case _:
            accept_seq_int(x)  # E: Argument 1 to "accept_seq_int" has incompatible type "T"; expected "Sequence[int]"

[github-actions]

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

[hauntsaninja]

Thanks! Would it make sense to move logic into construct_sequence_child?

[brianschubert]

Hmm, it probably would, though I'm hesitant to touch construct_sequence_child directly since it looks like it might need some more careful refactoring in the future to fix this TODO in its docstring:

mypy/mypy/checkpattern.py

Lines 750 to 751 in 1f200dd

	TODO: this doesn't make sense. For example if one has class S(Sequence[int], Generic[T])
	or class T(Sequence[Tuple[T, T]]), there is no way any of those can map to Sequence[str].

Changing less now might make that a bit easier.