title: Getting a list of indices where pandas boolean series is True
body: <p>I have a pandas series with boolean entries.  I would like to get a list of indices where the values are <code>True</code>.</p>
  
  <p>For example the input <code>pd.Series([True, False, True, True, False, False, False, True])</code></p>
  
  <p>should yield the output <code>[0,2,3,7]</code>.</p>
  
  <p>I can do it with a list comprehension, but is there something cleaner or faster?</p>