gh-124245: Fix UserWarning in test_argparse (GH-124246)

Author: serhiy-storchaka

Lib/test/test_argparse.py

@@ -5811,9 +5811,8 @@ def test_invalid_args(self):
         parser = ErrorRaisingArgumentParser(prog='PROG')
         parser.add_argument('--foo', nargs="*")
         parser.add_argument('foo')
-        with captured_stderr() as stderr:
+        with self.assertWarns(UserWarning):
             parser.parse_intermixed_args(['hello', '--foo'])
-            self.assertIn("UserWarning", stderr.getvalue())
 
 class TestIntermixedMessageContentError(TestCase):
     # case where Intermixed gives different error message