UnsetType union types lowering is incomplete

from typing import assert_type
from msgspec import UnsetType


def f(s: str | UnsetType):
    if s:
        assert_type(s, str)  # this fails

UNSET is the only UnsetType value and bool(UNSET) is False. Therefore the if statement should narrow the type of s to str.

I'd like to work on this. The fix is adding __bool__() -> Literal[False] to the UnsetType stub so type checkers can narrow via truthiness.

Fix UnsetType truthiness narrowing in type stubs

Ported to the community fork msgspec-arise: issue #26, PR #33

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