Hardly any changes at all, and the things that did change didn't actually make a clear difference in productivity or the clarity of your code. The ability to make breaking changes wasn't really used to improve the language, maybe except for unicode handling – no new syntax (would've loved functions-as-blocks myself), no getting rid of old crufty syntax, nada.
Python 2.7 supports string.format and Python 3 supports % formatting.
The only actual difference in that example is that print became a function in Python 3, requiring some additional parentheses. (Something that can be mechanically translated without much hassle.)
