title: Select xarray/pandas index based on specific months
body: <p>I have an xarray DataArray that I want to select the months April, May, June (similar to time.season=='JJA') for an entire time series.</p>
  
  <p>Its structured like: </p>
  
  <pre><code>&lt;xarray.DataArray 't2m' (time: 492, latitude: 81, longitude: 141)&gt;
  </code></pre>
  
  <p>I have been previously selecting JJA by:</p>
  
  <pre><code>seasonal_data =temp_data.sel(time=temp_data['time.season']=='JJA')
  </code></pre>
  
  <p>I would like to do the same thing but with the months 'AMJ' instead. I can add any details that I might be missing. </p>
  
  <p>Thanks</p>