Update keras_h5_conversion_test.py

[rthadur]

fixes #3611

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This change is 

[pyu10055]

thanks

Reviewable status: 0 of 1 approvals obtained (waiting on @pyu10055 and @rthadur)

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tfjs-converter/python/tensorflowjs/converters/keras_h5_conversion_test.py, line 172 at r1 (raw file):

    h5_path = os.path.join(self._tmp_dir, 'MyModelMerged.h5')
    model.save(h5_path)
    if six.PY3:

this is failing for python 2.x
E:172, 7: Undefined variable 'six' (undefined-variable)
E:221, 7: Undefined variable 'six' (undefined-variable)