Update left tests in test_subclassing for the opset version issue. (#2034)

Signed-off-by: Jay Zhang <[email protected]>

Author: fatcat-z

tests/keras2onnx_unit_tests/test_subclassing.py

@@ -4,7 +4,7 @@
 import mock_keras2onnx
 import numpy as np
 import tensorflow as tf
-from tf2onnx.keras2onnx_api import convert_keras
+from test_utils import convert_keras_for_test as convert_keras
 from mock_keras2onnx.proto import is_tensorflow_older_than
 
 if (not mock_keras2onnx.proto.is_tf_keras) or (not mock_keras2onnx.proto.tfcompat.is_tf2):