title: Provide __classcell__ example for Python 3.6 metaclass
body: <p>Per the 3.6.0 docs:</p>
  
  <blockquote>
    <p><strong>CPython implementation detail</strong>: In CPython 3.6 and later, the <code>__class__</code>
    cell is passed to the metaclass as a <code>__classcell__</code> entry in the class
    namespace. If present, this must be propagated up to the <code>type.__new__</code>
    call in order for the class to be initialized correctly. Failing to do
    so will result in a <code>DeprecationWarning</code> in Python 3.6, and a
    <code>RuntimeWarning</code> in the future.</p>
  </blockquote>
  
  <p>Can someone provide an example of doing this correctly?</p>
  
  <p>An example where it's actually needed?</p>