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

diff --git a/Lib/test/test_asyncgen.py b/Lib/test/test_asyncgen.py
index 62bf877..1f7e05b 100644
--- a/Lib/test/test_asyncgen.py
+++ b/Lib/test/test_asyncgen.py
@@ -2,7 +2,7 @@
 import types
 import unittest
 
-from test.support import import_module
+from test.support.import_helper import import_module
 asyncio = import_module("asyncio")