Fix for multiprocessing.queues.Queue not closing properly if no put.

See pypy issue #3372 and CPython issue #42752 for context.

This fixes the fact that close won't close the write pipe unless something had
already been put in the Queue.

--HG--
branch : py3_7_mp_queue_close_fix
2 jobs for branch/py3_7_mp_queue_close_fix in 8 minutes and 6 seconds (queued for 1 second)
Status Name Job ID Coverage
  Test
passed check_rpython_annotations #143101

00:07:05

passed hpy_tests #143102

00:08:06