bpo-31970: Reduce performance overhead of asyncio debug mode. (#4314)

* bpo-31970: Reduce performance overhead of asyncio debug mode.
diff --git a/Lib/asyncio/futures.py b/Lib/asyncio/futures.py
index 3dc9c500..472f2a8 100644
--- a/Lib/asyncio/futures.py
+++ b/Lib/asyncio/futures.py
@@ -79,7 +79,7 @@
             self._loop = loop
         self._callbacks = []
         if self._loop.get_debug():
-            self._source_traceback = traceback.extract_stack(sys._getframe(1))
+            self._source_traceback = events.extract_stack(sys._getframe(1))
 
     _repr_info = base_futures._future_repr_info