title: Filter dataframe group by not match to set
body: <p>Given this dataframe and set:</p>
  <pre><code>df = pd.DataFrame({'blah':['cats','cats','dogs','dogs','snakes','snakes','bears','bears'],
                     'foo':['A', 'B', 'B', 'A', 'A', 'B', 'A', 'A']})
     
      blah    foo
  0   cats    A
  1   cats    B
  2   dogs    B
  3   dogs    A
  4   snakes  A
  5   snakes  B
  6   bears   A
  7   bears   A
  
  letters = set(['A', 'B'])
  
  {'A', 'B'}
  </code></pre>
  <p>I want to filter the dataframe so that it includes only groups of <code>blah</code> where <code>set(foo)</code> does not match <code>letters</code>.</p>
  <p>My expected result is:</p>
  <pre><code>    blah    foo
  6   bears   A
  7   bears   A
  </code></pre>
  <p>Below is the closest thing I've got, but I can't figure out how to compare to <code>letters</code>, and I don't want to return a DataFrame and not a Series.</p>
  <pre><code>df.groupby('blah')['foo'].apply(set)
  </code></pre>