bpo-40275: Use new test.support helper submodules in tests (GH-21448)

diff --git a/Lib/test/test_imghdr.py b/Lib/test/test_imghdr.py
index 476ba95..b2d1fc8 100644
--- a/Lib/test/test_imghdr.py
+++ b/Lib/test/test_imghdr.py
@@ -4,7 +4,9 @@
 import pathlib
 import unittest
 import warnings
-from test.support import findfile, TESTFN, unlink
+from test.support import findfile
+from test.support.os_helper import TESTFN, unlink
+
 
 TEST_FILES = (
     ('python.png', 'png'),