Add true multiple inheritance to OOP lecture

[john-science]

Either in the second OOP lecture, or in the "More Examples" sub-lecture, we should allow for "true" multiple inheritance:

class Class1:
    def m(self):
        print("In Class1") 

class Class2(Class1):
    def m(self):
        print("In Class2")

class Class3:
    def x(self):
        print("In Class3")  

class Class4(Class2, Class3):
    pass   

c4 = Class4()
c4.m()  # prints: "In Class 2"
c4.x()  # prints: "In Class 3"