pep8
fix an infinite recursion on zmq emulation

zmq.sugar.context.Context.__init__ calls super(Context, self).__init__()
which sees greenhouse.emulation.zmq.ZMQSugarContext as the "Context"
class, which has the original Context class as a parent class, and leads
to the recursion.

this change leaves the greenhouse-emulated Context object as zmq.Context
and zmq.context.Context, it only removes it from that one class