bpo-30508: Don't log exceptions if Task/Future "cancel()" method called (#2050)

Lib/asyncio/futures.py

@@ -107,6 +107,7 @@ class Future:
         change the future's state to cancelled, schedule the callbacks and
         return True.
         """
+        self._log_traceback = False
         if self._state != _PENDING:
             return False
         self._state = _CANCELLED

Lib/asyncio/tasks.py

@@ -144,6 +144,7 @@ class Task(futures.Future):
         terminates with a CancelledError exception (even if cancel()
         was not called).
         """
+        self._log_traceback = False
         if self.done():
             return False
         if self._fut_waiter is not None: