Fixed tests for issue #19279.

Lib/test/test_codecs.py

@@ -840,7 +840,6 @@ class UTF7Test(ReadTest, unittest.TestCase):
             (b'a+////,+IKw-b', 'a\uffff\ufffd\u20acb'),
         ]
         for raw, expected in tests:
-            with self.subTest(raw=raw):
             self.assertRaises(UnicodeDecodeError, codecs.utf_7_decode,
                               raw, 'strict', True)
             self.assertEqual(raw.decode('utf-7', 'replace'), expected)