fix(argparse): match message with assertRaisesRegex

Author: mattwang44

Lib/test/test_argparse.py

@@ -797,10 +797,8 @@ def test_invalid_name(self):
 
     def test_prefix_chars_incompatibility(self):
         parser = argparse.ArgumentParser(prefix_chars='+')
-        with self.assertRaisesRegex(ValueError, ...) as cm:
+        with self.assertRaisesRegex(ValueError, "BooleanOptionalAction requires '-' in prefix_chars."):
             parser.add_argument('++foo', action=argparse.BooleanOptionalAction)
-        self.assertEqual(str(cm.exception),
-                         "BooleanOptionalAction requires '-' in prefix_chars.")
 
 class TestBooleanOptionalActionRequired(ParserTestCase):
     """Tests BooleanOptionalAction required"""