bpo-34183: Fix running Lib/test/test_contextlib_async.py as a script. (GH-8381) (GH-8422)

(cherry picked from commit db8e3a1e4476620b2b5aaf57acfc3ef58a08213b)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>
diff --git a/Lib/test/test_contextlib_async.py b/Lib/test/test_contextlib_async.py
index 879ddbe..e34fc38 100644
--- a/Lib/test/test_contextlib_async.py
+++ b/Lib/test/test_contextlib_async.py
@@ -4,7 +4,7 @@
 from test import support
 import unittest
 
-from .test_contextlib import TestBaseExitStack
+from test.test_contextlib import TestBaseExitStack
 
 
 def _async_test(func):