[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_thread.py b/Lib/test/test_thread.py
index 62b57fa..4ae8a83 100644
--- a/Lib/test/test_thread.py
+++ b/Lib/test/test_thread.py
@@ -132,6 +132,7 @@ def task():
             del task
             while not done:
                 time.sleep(POLL_SLEEP)
+                support.gc_collect()  # For PyPy or other GCs.
             self.assertEqual(thread._count(), orig)
 
     def test_unraisable_exception(self):