[3.9] bpo-40360: Handle PendingDeprecationWarning in test_lib2to3. (GH-21694) (GH-21697)

(cherry picked from commit cadda52d97)

Co-authored-by: Karthikeyan Singaravelan <tir.karthi@gmail.com>

Lib/test/test_lib2to3.py

@@ -1,5 +1,8 @@
-from lib2to3.tests import load_tests
 import unittest
+from test.support import check_warnings
+
+with check_warnings(("", PendingDeprecationWarning)):
+    from lib2to3.tests import load_tests
 
 if __name__ == '__main__':
     unittest.main()