Add overloads for asyncio.sleep() (#7873)

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.

stdlib/asyncio/tasks.pyi

@@ -270,7 +270,10 @@ def run_coroutine_threadsafe(coro: _FutureT[_T], loop: AbstractEventLoop) -> con
 
 if sys.version_info >= (3, 10):
     def shield(arg: _FutureT[_T]) -> Future[_T]: ...
-    async def sleep(delay: float, result: _T = ...) -> _T: ...
+    @overload
+    async def sleep(delay: float) -> None: ...
+    @overload
+    async def sleep(delay: float, result: _T) -> _T: ...
     @overload
     async def wait(fs: Iterable[_FT], *, timeout: float | None = ..., return_when: str = ...) -> tuple[set[_FT], set[_FT]]: ...  # type: ignore[misc]
     @overload
@@ -281,7 +284,10 @@ if sys.version_info >= (3, 10):
 
 else:
     def shield(arg: _FutureT[_T], *, loop: AbstractEventLoop | None = ...) -> Future[_T]: ...
-    async def sleep(delay: float, result: _T = ..., *, loop: AbstractEventLoop | None = ...) -> _T: ...
+    @overload
+    async def sleep(delay: float, *, loop: AbstractEventLoop | None = ...) -> None: ...
+    @overload
+    async def sleep(delay: float, result: _T, *, loop: AbstractEventLoop | None = ...) -> _T: ...
     @overload
     async def wait(  # type: ignore[misc]
         fs: Iterable[_FT], *, loop: AbstractEventLoop | None = ..., timeout: float | None = ..., return_when: str = ...