Add missing slow_callback_duration attribute to AbstractEventLoop (#2340)

stdlib/3/asyncio/events.pyi

@@ -39,6 +39,7 @@ class AbstractServer:
     def wait_closed(self) -> Generator[Any, None, None]: ...
 
 class AbstractEventLoop(metaclass=ABCMeta):
+    slow_callback_duration: float = ...
     @abstractmethod
     def run_forever(self) -> None: ...