title: ImportError: No module named model_selection
body: <p>I am trying to use <code>train_test_split</code> function and write:</p>
  
  <pre><code>from sklearn.model_selection import train_test_split
  </code></pre>
  
  <p>and this causes </p>
  
  <pre><code>ImportError: No module named model_selection
  </code></pre>
  
  <p>Why? And how to overcome?</p>