gh-118660: Add second type parameter to (Async)ContextManager

[JelleZijlstra]

• Issue: Add additional optional type parameter to typing.ContextManager and typing.AbstractContextManager #118660

[JelleZijlstra]

cc @Daverball

[Daverball]

I would maybe put a little bit more emphasis on the new type parameter being optional. While it defaulting to bool | None implies that, it's maybe a little bit too subtle for user-facing documentation, especially for a new feature in 3.13.

We could also consider changing the notation from Generic[T_co, ExitT_co] to Generic[T_co, ExitT_co = bool | None], so its behavior can be inferred just from this header, similar to function signatures, even if that is not valid syntax, it's similar to PEP-695 notation, while retaining the implicit reference to Generic within the typing module.

[JelleZijlstra]

I considered that syntax with Generic but didn't use it here because it's a SyntaxError. Switching to PEP 695 syntax might be nice but I think it causes problems with Sphinx. In any case, this is something we can adjust after the beta.

[AlexWaygood]

yes, we have to use an old version of Sphinx to keep Linux redistributors happy, and old versions of Sphinx don't know about PEP-695 syntax so they do weird things with the syntax highlighting

[AlexWaygood]

LGTM. Some "optional" inline suggestions below that might alleviate @Daverball's concerns a bit: