Add python3 classifier to setup.py

[pklapperich]

By listing only the "Python" classifier in setup.py, this project is advertising lack of support for Python3 and causes it to show up on projects like py3readiness (which attempts to track the porting effort to Python3 in advance of Python2 EOL).

I think this project is properly classified by:

    'Programming Language :: Python',
    'Programming Language :: Python :: 2.7',
    'Programming Language :: Python :: 3',
    'Programming Language :: Python :: 3.4',
    'Programming Language :: Python :: 3.5',
    'Programming Language :: Python :: 3.6',
    'Programming Language :: Python :: 3.7',

[lebedov]

Good point - done.