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

diff --git a/Lib/test/test_fstring.py b/Lib/test/test_fstring.py
index 0dc7dd8..35a62a0 100644
--- a/Lib/test/test_fstring.py
+++ b/Lib/test/test_fstring.py
@@ -12,7 +12,7 @@
 import types
 import decimal
 import unittest
-from test.support import temp_cwd
+from test.support.os_helper import temp_cwd
 from test.support.script_helper import assert_python_failure
 
 a_global = 'global variable'