It's really important to be clear about the lessons to be learned from Python 3.

1. Forward compatibility is more important than backward compatibility. 2. Automated refactoring tools don't help with 1.

The problem wasn't that they broke a lot in Python 3. It was that you couldn't write your Python 2 in such a way as to be compatible with it until well into the transition process as the six package got popular and the devs fixed needlessly broken things in Python 2.