title: Insert element in Python list after every nth element
body: <p>Say I have a Python list like this:</p>
  
  <pre><code>letters = ['a','b','c','d','e','f','g','h','i','j']
  </code></pre>
  
  <p>I want to insert an 'x' after every nth element, let's say three characters in that list. The result should be:</p>
  
  <pre><code>letters = ['a','b','c','x','d','e','f','x','g','h','i','x','j']
  </code></pre>
  
  <p>I understand that I can do that with looping and inserting. What I'm actually looking for is a Pythonish-way, a one-liner maybe?</p>