libvirt-python: add classifiers to setup.py
libvirt-python: add classifiers to setup.py

Add the Python 3 classifier, needed by the caniusepython3 tool to check
if dependencies of a projects are Python 3 compatible:

    https://caniusepython3.com/

libvirt/libvirt-python,libvirt/libvirt-python,libvirt/libvirt-python