title: python: most elegant way to intersperse a list with an element
body: <p>Input:</p>
  
  <pre><code>intersperse(666, ["once", "upon", "a", 90, None, "time"])
  </code></pre>
  
  <p>Output:</p>
  
  <pre><code>["once", 666, "upon", 666, "a", 666, 90, 666, None, 666, "time"]
  </code></pre>
  
  <p>What's the most elegant (read: Pythonic) way to write <code>intersperse</code>?</p>