title: PEP8 [E713] test for membership should be 'not in'
body: <p>Given the following code: </p>
  
  <pre><code>d = {'a':1, 'b':2}
  
  if not 'c' in d:
   print 'kaboom'
  
  if 'c' not in 'd':
   print 'kaboom'
  </code></pre>
  
  <p>In both cases this prints 'kaboom'. What is the difference and why PEP8 complains about the former.</p>