Please read http://wiki.python.org/moin/Python2orPython3

Most "production code" runs on Python 2.x (although 2.7 would be better than the 2.6 that's currently in the N9 repos, as 2.7 has many features from 3.x backported, and also is still supported), so no need yet for Python 3 - especially since some libraries are not yet compatible with Python 3 or have only experimental support. If you want to use PySide (Qt) on the N9, for example, you should stick to Python 2.