Fix trove classifiers.

Quoted from:

    In order to show up on the list of projects that support Python 3,
    the project needs to set the Programming Language :: Python :: 3
    trove classifier.

http://nothingbutsnark.svbtle.com/my-experience-creating-caniusepython3
Merge pull request #4 from berkerpeksag/minor-updates

Fix trove classifiers.