title: __slots__ type annotations for Python / PyCharm
body: <p>How can I provide type annotations for attributes which are defined via <code>__slots__</code>? E.g. for this class:</p>
  
  <pre><code>class DiffMatch:
    __slots__ = ["ref_seq_idx", "ref_txt", "hyp_txt", "start_time", "end_time"]
  
    def __repr__(self):
      return "%s(%s)" % (self.__class__.__name__, ", ".join(["%s=%r" % (a, getattr(self, a)) for a in self.__slots__]))
  </code></pre>
  
  <p>(Python 2 compatible in a comment if possible (otherwise Python 3 only is fine), and also such that PyCharm can handle it (actually that is most important for me now).)</p>