Python 3 vs 2: if statement on objects and __nonzero__ [duplicate]

This question already has an answer here:

• Defining “boolness” of a class in python 3 answers
• In Python, what operator to override for “if object:”? 3 answers

I'm trying to port something from Python 2 to 3, and have run into something that appears to be an important difference, but that I'm having trouble finding information about by googling. Consider the following:

class TestClass:
    def __nonzero__(self):
        print("nonzero called")
        return True

instance = TestClass()
if instance:
    pass

If I run this code in Python 2, it prints "nonzero called", but if I run it in Python 3, it does not. So it seems like in Python 3, if statements on objects are evaluated some other way? What's going on here?

Thanks!