message: Fix the deprecation_test for py3.8
diff: @@ -761,7 +761,7 @@ class DeprecatedArgValuesTest(test.TestCase):
  deprecation.deprecated_arg_values(date, None, deprecated=True)
  with self.assertRaisesRegexp(ValueError, "instructions"):
  deprecation.deprecated_arg_values(date, "", deprecated=True)
  - with self.assertRaisesRegexp(ValueError, "argument", deprecated=True):
  + with self.assertRaisesRegexp(ValueError, "argument"):
  deprecation.deprecated_arg_values(date, instructions)
  @test.mock.patch.object(logging, "warning", autospec=True)
  