Fix Python 3 installation.

In Python 3, the long_description field should not be a bytes object.

I also cleanup the Python version classfiers.

Note: This issue can be reproduced by installing astor manually(e.g.
python3 setup.py install) or adding astor to install_requires in your
project(e.g. https://github.com/hylang/hy/pull/328/files)