bpo-31326: ProcessPoolExecutor waits for the call queue thread (#3265)

* bpo-31326: ProcessPoolExecutor waits for the call queue thread

concurrent.futures.ProcessPoolExecutor.shutdown() now explicitly
closes the call queue. Moreover, shutdown(wait=True) now also join
the call queue thread, to prevent leaking a dangling thread.

* Fix for shutdown() being called twice.
2 files changed