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

diff --git a/Lib/test/test_poplib.py b/Lib/test/test_poplib.py
index b670afc..2ac345d 100644
--- a/Lib/test/test_poplib.py
+++ b/Lib/test/test_poplib.py
@@ -15,6 +15,7 @@
 from test import support as test_support
 from test.support import hashlib_helper
 from test.support import socket_helper
+from test.support import threading_helper
 
 HOST = socket_helper.HOST
 PORT = 0
@@ -536,11 +537,11 @@
 def test_main():
     tests = [TestPOP3Class, TestTimeouts,
              TestPOP3_SSLClass, TestPOP3_TLSClass]
-    thread_info = test_support.threading_setup()
+    thread_info = threading_helper.threading_setup()
     try:
         test_support.run_unittest(*tests)
     finally:
-        test_support.threading_cleanup(*thread_info)
+        threading_helper.threading_cleanup(*thread_info)
 
 
 if __name__ == '__main__':