bpo-30301: Fix AttributeError when using SimpleQueue.empty() (#1601) (#1628)

Under *spawn* and *forkserver* start methods, SimpleQueue.empty() could
raise AttributeError due to not setting _poll in __setstate__.

Lib/multiprocessing/queues.py

@@ -337,6 +337,7 @@ class SimpleQueue(object):
 
     def __setstate__(self, state):
         (self._reader, self._writer, self._rlock, self._wlock) = state
+        self._poll = self._reader.poll
 
     def get(self):
         with self._rlock: