title: Pandas groupby two columns then get dict for values
body: <p>I have a pandas dataframe:</p>
  
  <pre><code>banned_titles = 
  TitleId  RelatedTitleId
  0    89989           32598
  1    89989         3085083
  2    95281         3085083
  </code></pre>
  
  <p>when I apply groupby as following</p>
  
  <pre><code>In [84]: banned_titles.groupby('TitleId').groups
  Out[84]: {89989: [0, 1], 95281: [2]}
  </code></pre>
  
  <p>This is so close but not I want. </p>
  
  <p>What I want is:</p>
  
  <pre><code>{89989: [32598, 3085083], 95281: [3085083]}
  </code></pre>
  
  <p>Is there a way to do this?</p>