simplify EUI.eui64, add test for EUI64 -> EUI64 for py 2.x and 3.x

converting EUI -> str -> EUI seems a bit much if just a new EUI object of same value is wanted

also, why EUI() constructor call in one case and self.__class__() constructor call in the other?
EUI: add ipv6 and modified_eui64 methods, add tests