title: Pandas .dt.hour formatting
body: <p>I have a <code>datetime64[ns]</code> format object in a pandas dataframe.</p>
  
  <p>I can use this column to compute the hour via:</p>
  
  <pre><code>df['hour'] = df['datetime'].dt.hour
  </code></pre>
  
  <p>This returns an integer. e.g :</p>
  
  <pre><code>datetime                        hour
  19-10-2015 2015-10-19 01:00:00  1  
  </code></pre>
  
  <p>What is the easiest way to output the <code>hour</code> column in the <code>01:00:00</code> time format?</p>