Follow updates on pyzmq 13.0.0

 - XREP/XREQ constants are now definitively replaced by ROUTER/DEALER.
 - Some weird getattr/setattr magic were added to pyzmq, which makes
   inheriting from pyzmq classes annoyingly frustrating, especially:
	- zmq.Context: solved by defined attributes wiht None, at the class
	  level.
	- zmq.Socket: solved by using self.__dict__ directly.