bpo-30886: Fix multiprocessing.Queue.join_thread() (#2642)

multiprocessing.Queue.join_thread() now waits until the thread
completes, even if the thread was started by the same process which
created the queue.

Fix the following warning which occurs randomly when running
test_handle_called_with_mp_queue of test_logging.QueueListenerTest:

Warning -- threading_cleanup() failed to cleanup -1 threads after 4 sec (count: 0, dangling: 1)