bpo-39349: Add cancel_futures to Executor.shutdown base class (python…

…GH-22023)

* Add cancel_futures parameter to the Executor base class, since it was missed in the original PR (python#18057) that added cancel_futures.

Lib/concurrent/futures/_base.py

@@ -605,7 +605,7 @@ def result_iterator():
                     future.cancel()
         return result_iterator()
 
-    def shutdown(self, wait=True):
+    def shutdown(self, wait=True, *, cancel_futures=False):
         """Clean-up the resources associated with the Executor.
 
         It is safe to call this method several times. Otherwise, no other
@@ -615,6 +615,9 @@ def shutdown(self, wait=True):
             wait: If True then shutdown will not return until all running
                 futures have finished executing and the resources used by the
                 executor have been reclaimed.
+            cancel_futures: If True then shutdown will cancel all pending
+                futures. Futures that are completed or running will not be
+                cancelled.
         """
         pass