title: Get the pandas dtype representation of interval
body: <p>I've written some code that converts some input data to pandas data frame, making sure that date &amp; time fields are converted to the relevant data type (Timestamp and Timedelta). </p>
  
  <p>In my tests I'm writting asserts for the dtypes of the columns, it look like that:</p>
  
  <pre><code>test_data_frame = input_data_to_df(test_data) # type: pd.DataFrame
  
  self.assertEquals(
      pandas.core.dtypes.dtype.DatetimeTZDtype.construct_from_string('datetime64[ns, UTC]')
      test_data_frame['timestamp_column'].dtype)
  </code></pre>
  
  <p>This works well fo column of type <code>datetime64[ns, UTC]</code> or any <code>datetime64</code></p>
  
  <p>I'm trying to do the same thing with a column of type <code>timedelta64[ns]</code> but I can't get  the relevant dtype representation.</p>
  
  <ul>
  <li>This doesn't work <code>DatetimeTZDtype.construct_from_string('timedelta64[ns]')</code></li>
  <li><code>IntervalDtype.construct_from_string('timedelta64[ns]')</code> returns an <code>interval[timedelta64[ns]]</code> which isn't what I want</li>
  </ul>