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

diff --git a/Lib/test/test_tk.py b/Lib/test/test_tk.py
index 48cefd9..59842a5 100644
--- a/Lib/test/test_tk.py
+++ b/Lib/test/test_tk.py
@@ -1,6 +1,7 @@
 from test import support
+from test.support import import_helper
 # Skip test if _tkinter wasn't built.
-support.import_module('_tkinter')
+import_helper.import_module('_tkinter')
 
 # Skip test if tk cannot be initialized.
 support.requires('gui')