bpo-36813: Fix QueueListener to call task_done() upon termination. (GH-13113)

Fixed QueueListener in order to avoid random deadlocks.
Unable to add regression tests atm due to time constraints, will add it in a bit.
Regarding implementation, although it's nested, it does not cause performance issues whatsoever, and does not call task_done() in case of an exception (which is the right thing to do IMHO).


https://bugs.python.org/issue36813

Lib/logging/handlers.py

@@ -1477,6 +1477,8 @@ class QueueListener(object):
             try:
                 record = self.dequeue(True)
                 if record is self._sentinel:
+                    if has_task_done:
+                        q.task_done()
                     break
                 self.handle(record)
                 if has_task_done: