Relax test timing (bpo-29861) to avoid sporadic failures (#1120) (#1472)

(cherry picked from commit 685cdb9)

Author: vstinner

Lib/test/test_multiprocessing.py

@@ -1290,6 +1290,7 @@ def test_release_task_refs(self):
         self.pool.map(identity, objs)
 
         del objs
+        time.sleep(DELTA)  # let threaded cleanup code run
         self.assertEqual(set(wr() for wr in refs), {None})
         # With a process pool, copies of the objects are returned, check
         # they were released too.