classes was called with three arguments.  This makes no sense, there's
no way to pass in the "modulo" 3rd argument as for __pow__, and
classic classes don't do this.  [SF bug 620179]

I don't want to backport this to 2.2.2, because it could break
existing code that has developed a work-around.  Code in 2.2.2 that
wants to use __ipow__ and wants to be forward compatible with 2.3
should be written like this:

  def __ipow__(self, exponent, modulo=None):
      ...