asyncio.sleep() should have an overload

[wch]

This is from microsoft/pyright#3475.

With the latest version of pyright (1.1.247), this code results in an error:

from typing import Awaitable, TypeVar
import asyncio
T = TypeVar("T")

async def wrap(coro: Awaitable[T]) -> T:
    return await coro

async def wrap_sleep() -> None:
    await wrap(asyncio.sleep(0))

The result when I run pyright on it:

Found 1 source file
/..../test/test.py
  /..../test/test.py:9:16 - error: Argument type is partially unknown
    Argument corresponds to parameter "coro" in function "wrap"
    Argument type is "Coroutine[Any, Any, Unknown]" (reportUnknownArgumentType)
1 error, 0 warnings, 0 informations 

The fix, as suggested by @JelleZijlstra and @erictraut, is to add an overload like this:

 async def sleep(delay: float) -> None: ...

[srittau]

Sounds right, PR welcome!

Side note: This could benefit from python/typing#307.