bpo-40275: Use new test.support helper submodules in tests (GH-21315)

diff --git a/Lib/test/test_cgitb.py b/Lib/test/test_cgitb.py
index bab152d..590ffde 100644
--- a/Lib/test/test_cgitb.py
+++ b/Lib/test/test_cgitb.py
@@ -1,4 +1,4 @@
-from test.support import temp_dir
+from test.support.os_helper import temp_dir
 from test.support.script_helper import assert_python_failure
 import unittest
 import sys