Add Python 3 Trove classifier

Add the Python 3 Trove classifier (`"Programming Language :: Python :: 3"`) to `setup.py`.

This advertises that the package is Python 3 compatible and is for the benefit of prospective users and tools that crawl PyPI and collect information about which packages are Python 3 compatible (e.g.: http://python3wos.appspot.com/). This also will make PyStemmer show up on the ["Python 3 Packages" list at PyPI](https://pypi.python.org/pypi?:action=browse&c=533&show=all).

I also aded a few more Trove classifiers for specific versions of Python. I'm not sure what these are used for beyond display in PyPI for prospective users, but I've seen other packages use them.