title: Get index of series where value is True
body: <p>How do I select only True values?</p>
  
  <pre><code>myindex=['a', 'b', 'c' , 'd']
  myseries=pd.Series([True, True, False, True], index=myindex)
  
  a     True
  b     True
  c    False
  d     True
  dtype: bool
  </code></pre>
  
  <p>What I have tried:</p>
  
  <pre><code>myseries.where(myseries == True)
  </code></pre>
  
  <p>This includes "c" while I need to return a list of a, b and d</p>