[3.10] bpo-25130: Add calls of gc.collect() in tests to support PyPy (GH-28005) (GH-28027)

(cherry picked from commit 2a8127cafe1d196f858a3ecabf5f1df3eebf9a12)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>
diff --git a/Lib/test/test_deque.py b/Lib/test/test_deque.py
index 8bd6ebd..7886bbf 100644
--- a/Lib/test/test_deque.py
+++ b/Lib/test/test_deque.py
@@ -869,6 +869,7 @@ def test_weakref(self):
         p = weakref.proxy(d)
         self.assertEqual(str(p), str(d))
         d = None
+        support.gc_collect()  # For PyPy or other GCs.
         self.assertRaises(ReferenceError, str, p)
 
     def test_strange_subclass(self):