message: Replace assert_called_once, which is not available for < python 3.6
diff: @@ -315,7 +315,7 @@ class XlaDecoratorTest(test.TestCase, parameterized.TestCase):
  estimator_spec = model_fn(
  features=features, labels=labels, mode=_TRAIN, params=params or {})
  - mock_xla_compile.assert_called_once()
  + self.assertEqual(mock_xla_compile.call_count, 1)
  self.assertEqual(estimator_spec.mode, _TRAIN)
  with self.test_session() as sess:
  