[Python-3000] py3k keyword is gone

["Martin v. Löwis"]

As people had disliked the py3k keyword in the tracker,
I have now changed all items that were still using it to
use the Python 3.0 version instead, and retired that keyword.

So please search for version "Python 3.0" if you want to see
issues related to Python 3000.

Regards,
Martin