Add overloads for asyncio.sleep().

[wch]

Closes #7866. This adds an overload to asyncio.sleep(), so that when it is called without return=None, the type checker knows that the return type is None instead of unknown.

Also related to microsoft/pyright#3475.

[wch]

Looking for a bit of advice. This part fails the CI checks on Python <= 3.9. (Python >= 3.10 uses a different code path which seems to be fine.)

    @overload
    async def sleep(delay: float, *, loop: AbstractEventLoop | None = ...) -> None: ...
    @overload
    async def sleep(delay: float, result: _T = ..., *, loop: AbstractEventLoop | None = ...) -> _T: ...

stdlib/asyncio/tasks.pyi:288: error: Overloaded function signatures 1 and 2 overlap with incompatible return types  [misc]

Is it possible to make this work?

[AlexWaygood]

Is it possible to make this work?

Committing @JelleZijlstra's suggestions he made in review just now should do it :)

[wch]

@AlexWaygood Thanks, he must have posted that as I was typing. :)

[github-actions]

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