bpo-40275: Adding threading_helper submodule in test.support (GH-20263)

diff --git a/Lib/test/test_threadedtempfile.py b/Lib/test/test_threadedtempfile.py
index e1d7a10..fe63c9e 100644
--- a/Lib/test/test_threadedtempfile.py
+++ b/Lib/test/test_threadedtempfile.py
@@ -15,7 +15,7 @@
 
 import tempfile
 
-from test.support import start_threads
+from test.support import threading_helper
 import unittest
 import io
 import threading
@@ -50,7 +50,7 @@
 class ThreadedTempFileTest(unittest.TestCase):
     def test_main(self):
         threads = [TempFileGreedy() for i in range(NUM_THREADS)]
-        with start_threads(threads, startEvent.set):
+        with threading_helper.start_threads(threads, startEvent.set):
             pass
         ok = sum(t.ok_count for t in threads)
         errors = [str(t.name) + str(t.errors.getvalue())