bpo-34037: Fix test_asyncio failure and add loop.shutdown_default_executor() (GH-15735)

diff --git a/Lib/asyncio/events.py b/Lib/asyncio/events.py
index 5fb5464..2f06c4a 100644
--- a/Lib/asyncio/events.py
+++ b/Lib/asyncio/events.py
@@ -249,6 +249,10 @@
         """Shutdown all active asynchronous generators."""
         raise NotImplementedError
 
+    async def shutdown_default_executor(self):
+        """Schedule the shutdown of the default executor."""
+        raise NotImplementedError
+
     # Methods scheduling callbacks.  All these return Handles.
 
     def _timer_handle_cancelled(self, handle):