Implement upstream additional_classifiers

As part of preparing for #15 , we should modify our templates and yaml so that they use the same features as upstream.

One such feature is setup.py.jj2's additional_classifiers

https://github.com/moremoban/pypi-mobans/blob/master/templates/setup.py.jj2#L82

This should be done instead of the hard-coded list at https://gitlab.com/coala/mobans/blob/master/templates/coala-setup.py.jj2#L187