fix: narrow scope to Python 3.13+ only

Revert changes to <3.13 stubs to avoid breaking existing code that
assigns as_completed() sync-iter results to Future/Task variables.
The Coroutine type is correct at runtime but too breaking for older versions.

Only 3.13+ gets the fix where _SyncAndAsyncIterator Protocol exists.

Author: armorbreak001

stdlib/asyncio/tasks.pyi

@@ -93,12 +93,12 @@ if sys.version_info >= (3, 13):
     def as_completed(fs: Iterable[_FutureLike[_T]], *, timeout: float | None = None) -> _SyncAndAsyncIterator[_T]: ...
 
 elif sys.version_info >= (3, 10):
-    def as_completed(fs: Iterable[_FutureLike[_T]], *, timeout: float | None = None) -> Iterator[Coroutine[Any, Any, _T]]: ...
+    def as_completed(fs: Iterable[_FutureLike[_T]], *, timeout: float | None = None) -> Iterator[Future[_T]]: ...
 
 else:
     def as_completed(
         fs: Iterable[_FutureLike[_T]], *, loop: AbstractEventLoop | None = None, timeout: float | None = None
-    ) -> Iterator[Coroutine[Any, Any, _T]]: ...
+    ) -> Iterator[Future[_T]]: ...
 
 @overload
 def ensure_future(coro_or_future: _FT, *, loop: AbstractEventLoop | None = None) -> _FT: ...  # type: ignore[overload-overlap]