Add python 2 and python 3 classifiers in the setup.py file.

`Flask-Restless` is incorrectly considered not compatible with python 3 by several tools like `caniusepython3.com` : https://caniusepython3.com/project/Flask-Restless

I added the correct classifiers in the `setup.py` file as described here : https://docs.python.org/3/howto/pyporting.html#before-you-begin