[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_set.py b/Lib/test/test_set.py
index b1fab0f..29bb39d 100644
--- a/Lib/test/test_set.py
+++ b/Lib/test/test_set.py
@@ -593,6 +593,7 @@ def test_weakref(self):
         p = weakref.proxy(s)
         self.assertEqual(str(p), str(s))
         s = None
+        support.gc_collect()  # For PyPy or other GCs.
         self.assertRaises(ReferenceError, str, p)
 
     def test_rich_compare(self):