Add Python 3 only classifier

Python 2 support was removed during Ussuri cycle. This change adds
the classifier to clearly state that only Python 3 is supported.

Note that the python 3 only classifier was already added to the other
oslo libraries so this improves consistency among all libraries.

Change-Id: I961f128f9aaaf82cc1345a0c19bdfbae516f8f43