PEP 742 (TypeIs) tracker

[JelleZijlstra]

This issue is supposed to track when we can start to use the TypeIs special form (PEP 742) in typeshed. Support is needed in released versions of:

• PEP accepted
• mypy (Implement TypeIs (PEP 742) mypy#16898)
• pytype (Support PEP 742 (TypeIs) google/pytype#1588)
• pyright (Implement support for TypeIs (PEP 742) microsoft/pyright#7293)
• pyre (optional)
• PyCharm (optional)

Please reference this issue as needed from other issues/PRs to collect places that we might want to update.

Once this is supported, many (most?) TypeGuard functions in typeshed should be changed to use TypeIs.

[sterliakov]

Following #11823, should all also become a TypeIs guard against None elements?

def all(iterable: Iterable[_T | None], /) -> TypeIs[Iterable[_T]]: ...

This wasn't possible with TypeGuard due to undesired type narrowing (iterable type would be lost, Sequence[int | None] would become Iterable[int]), but with TypeIs intersection semantics that problem should not arise (Sequence[int| None] & Iterable[int] should be reducible to Sequence[int], if I'm not mistaken, which is the desirable outcome).

[JelleZijlstra]

Intuitively that seems fine but both mypy and pyright have some issues with this (e.g., they seem to reduce list[str | None] & Iterable[str] to Never): https://pyright-play.net/?pyrightVersion=1.1.359&pythonVersion=3.13&code=GYJw9gtgBALgngBwJYDsDmUkQWEMogCmAboQIYA2A%2BvAoQDRQCSMhIZARhYQLABQoSLESo0VQgA9WKAM5IwszNlz4AKokJMZ-fgBNCwKJQoBtKqoC6ACiSt2XQgC5mdztzOqoAHygA5BYQWjAD0AJRQALQAfFDqdFomLGxuhB4WFs4AdNk6fPqGMmTY3FYSzkn27jIwIN5%2BAUFQcM4USNUm1bU%2B-iiBoY78UEOYhsal-YPDU0SklDQa45NDhBQyTktTBCTk1LSEi3xTSKMUFFZwE4ebQzM783TnoRsrawNX17dze49AA, https://mypy-play.net/?mypy=latest&python=3.12&gist=b4a3be40d6e245ac7e65ff9ffac03355

Worth trying with an exploratory PR, and maybe with tweaks to the intersection logic in mypy and/or pyright.

Anyway, I'm closing this PR as we can use TypeIs in typeshed now. Feel free to open specific issues about places where TypeIs could be used.