A simple change in Num2 class like this:
super().__init__(num)
It works in python3.
class Num:
def __init__(self,num):
self.n1 = num
class Num2(Num):
def __init__(self,num):
super().__init__(num)
self.n2 = num*2
def show(self):
print (self.n1,self.n2)
mynumber = Num2(8)
mynumber.show()