title: python rearrange nested list
body: <p>given</p>
  <pre><code>[[1,2,3], [4,5,6], [7,8,9]]
  </code></pre>
  <p>I'd like to get</p>
  <pre><code>[[1,4,7], [2,5,8], [3,6,9]]
  </code></pre>
  <p>So I'd like to group <code>nth</code> item into <code>nth</code> resulting list.</p>