[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_functools.py b/Lib/test/test_functools.py
index fbf5578..fece825 100644
--- a/Lib/test/test_functools.py
+++ b/Lib/test/test_functools.py
@@ -167,6 +167,7 @@ def test_weakref(self):
         p = proxy(f)
         self.assertEqual(f.func, p.func)
         f = None
+        support.gc_collect()  # For PyPy or other GCs.
         self.assertRaises(ReferenceError, getattr, p, 'func')
 
     def test_with_bound_and_unbound_methods(self):