Add trove classifiers specifying Python 3 support

In *setup.py*, I added [trove classifiers](https://pypi.python.org/pypi?%3Aaction=list_classifiers) to clarify that the package supports Python 3. Besides PyPI, this information is used by [caniusepython3](https://github.com/brettcannon/caniusepython3) and sites like http://py3readiness.org to determine if a package is Py3 ready.

I was conservative with the classifiers and didn't, for example, add information about the license or development status, though this information would also be useful to include.