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