class members invalid behaviour on python 3

Take the following code (the Test class is wrapped by shiboken generator):

>>>
from Test import TestType

class MyObject(TestType):
Instance = "Default"

MyObject.Instance = "First"
print(MyObject.Instance)
MyObject.Instance = "Second"
print(MyObject.Instance)
<<<

Valid output on python 2:
First
Second

Invalid output on python 3:
First
First