PyPI classifiers, 2.6 compatibility

I added PyPI trove classifiers for tested python versions. This allows tools such as [caniusepython3](https://pypi.python.org/pypi/caniusepython3) to properly detect python 3 support.

Also https://github.com/ahupp/python-magic/commit/a5ebf0d558c3a851c0884d30a0051836f7db0c2a broke python 2.6 so I fixed it.

After merging it'd be useful to run `setup.py register` so that PyPI picks up the newly added classifiers.

Thanks!