message: update python_api.md
  This fixes
  Changed `tf.gfile.GFile` with `tf.io.gfile.GFile` for the code to work in TF 2.2
  The former is an alias declared in TF 1.X and fails in TF 2 due to absence.
diff: @@ -93,7 +93,7 @@ converter = tf.lite.TFLiteConverter.from_keras_model(model)
  tflite_model = converter.convert()
  # Save the TF Lite model.
  -with tf.gfile.GFile('model.tflite', 'wb') as f:
  +with tf.io.gfile.GFile('model.tflite', 'wb') as f:
  f.write(tflite_model)
  ```
  