title: Pandas conditional groupby
body: <p>I have a dataframe like below:</p>
  
  <pre><code>df = pd.DataFrame({'col_1': [6ai,6aii,6aii,6b],
                 'col_2': [1,1,5,1],
                 'col_3':[True,False,True,False]})
  
     col_1  col_2 col_3
  0    6a1      1    True
  1    6aii     1    False
  2    6aii     5    True
  3    6b       1    False
  </code></pre>
  
  <p>I want to group this dataframe on col_1, and then only select the row where col_3 is True. In cases where I have only one occurrence of a value in col_1, I want to select the row regardless col_3 is True or False.
  So the result I'm after is:</p>
  
  <pre><code>   col_1  col_2 col_3
  0    6a1      1    True
  2    6aii     5    True
  3    6b       1    False
  </code></pre>
  
  <p>I'm thinking that I should use groupby, but I'm not sure. I could really use some help please?</p>