[asyncio.tasks] Fix return type of as_completed (#15672)

When as_completed() is iterated with a regular for loop (not async
for), it yields coroutines, not Futures. The previous stub incorrectly
typed the sync iterator path as Iterator[Future[_T]] which would
allow calling .result() without a type error even though that crashes
at runtime with AttributeError.

Author: armorbreak001

stdlib/asyncio/tasks.pyi

@@ -87,9 +87,9 @@ ALL_COMPLETED: Final = concurrent.futures.ALL_COMPLETED
 
 if sys.version_info >= (3, 13):
     @type_check_only
-    class _SyncAndAsyncIterator(Iterator[_T_co], AsyncIterator[_T_co], Protocol[_T_co]): ...
+    class _SyncAndAsyncIterator(Iterator[Coroutine[Any, Any, _T]], AsyncIterator[Future[_T]], Protocol[_T]): ...
 
-    def as_completed(fs: Iterable[_FutureLike[_T]], *, timeout: float | None = None) -> _SyncAndAsyncIterator[Future[_T]]: ...
+    def as_completed(fs: Iterable[_FutureLike[_T]], *, timeout: float | None = None) -> _SyncAndAsyncIterator[_T]: ...
 
 else:
     def as_completed(fs: Iterable[_FutureLike[_T]], *, timeout: float | None = None) -> Iterator[Future[_T]]: ...