bpo-30966: Add multiprocessing.SimpleQueue.close() (GH-19735)

Add a new close() method to multiprocessing.SimpleQueue to explicitly
close the queue.

Automerge-Triggered-By: @pitrou

Lib/multiprocessing/queues.py

@@ -346,6 +346,10 @@ class SimpleQueue(object):
         else:
             self._wlock = ctx.Lock()
 
+    def close(self):
+        self._reader.close()
+        self._writer.close()
+
     def empty(self):
         return not self._poll()