add asyncio.__all__ (#13038)

stdlib/asyncio/tasks.pyi

@@ -18,6 +18,7 @@ from .futures import Future
 if sys.version_info >= (3, 11):
     from contextvars import Context
 
+# Keep asyncio.__all__ updated with any changes to __all__ here
 if sys.version_info >= (3, 12):
     __all__ = (
         "Task",