title: timedelta to string type in pandas dataframe
body: <p>I have a dataframe <code>df</code> and its first column is <code>timedelta64</code></p>
  
  <pre><code>df.info():
  
  &lt;class 'pandas.core.frame.DataFrame'&gt;
  RangeIndex: 686 entries, 0 to 685
  Data columns (total 6 columns):
  0    686 non-null timedelta64[ns]
  1    686 non-null object
  2    686 non-null object
  3    686 non-null object
  4    686 non-null object
  5    686 non-null object
  </code></pre>
  
  <p>If I <code>print(df[0][2])</code>, for example, it will give me <code>0 days 05:01:11</code>. However, I don't want the <code>0 days</code> filed. I only want <code>05:01:11</code> to be printed. Could someone teaches me how to do this? Thanks so much!</p>