Issue #11815: Use a light-weight SimpleQueue for the result queue in concurrent.futures.ProcessPoolExecutor.

cannot raise queue.Empty).

cannot raise queue.Empty).

worker processes: new processes would be spawned while the pool is being
shut down.  Patch by Charles-François Natali.

Eventually I'll actually replace the calls in the tests themselves.

and on OS X with pydebug.

is passed (previously, a select.error with EINVAL would be raised).  Patch
by Charles-François Natali.