message: fix iris example to work with python3
  iris.py did not work with python3 as urllib.urlopen is not in python3.
  Switched to urlretrive from six. Same was done in:
  tensorflow/examples/image_retraining/retrain.py
diff: @@ -21,7 +21,8 @@ from __future__ import division
  from __future__ import print_function
  import os
  -import urllib
  +
  +from six.moves.urllib.request import urlretrieve
  import tensorflow as tf
  @@ -38,9 +39,7 @@ FEATURE_KEYS = ['sepal_length', 'sepal_width', 'petal_length', 'petal_width']
  def maybe_download_iris_data(file_name, download_url):
  """Downloads the file and returns the number of data."""
  if not os.path.exists(file_name):
  - raw = urllib.urlopen(download_url).read()
  - with open(file_name, 'w') as f:
  - f.write(raw)
  + urlretrieve(download_url, file_name)
  # The first line is a comma-separated string. The first one is the number of
  # total data in the file.
  