title: Order by DateTimeRange field on a django model
body: <p>In my django project, I have a model with a Postgres DateTimeRangeField holding the time lapse between the start and the end of a long task.
  Something like this:</p>
  
  <pre><code>timespan = DateTimeRangeField()
  </code></pre>
  
  <p>Is it possible to use order_by on the queryset using either the lower or upper attributes of the range field?</p>