Part of my Python FAQ, which is doomed to never be finished.
Maybe you have a Python 2 codebase. Maybe you’d like to make it work with Python 3. Maybe you really wish someone would write a comically long article on how to make that happen.
I have good news! You’re already reading one.
(And if you’re not sure why you’d want to use Python 3 in the first place, perhaps you’d be interested in the companion article which delves into exactly that question?)