Be explicit about supported Python versions

[Hugo]

src/setup.py contains these classifiers:

        'Programming Language :: Python',
        'Programming Language :: Python :: 3',

It would be good to explicitly state which versions are supported, to allow scripts to automatically determine the compatibility of pytz.

For example:

        'Programming Language :: Python',
        'Programming Language :: Python :: 2',
        'Programming Language :: Python :: 2.4',
        'Programming Language :: Python :: 2.5',
        'Programming Language :: Python :: 2.6',
        'Programming Language :: Python :: 2.7',
        'Programming Language :: Python :: 3',
        'Programming Language :: Python :: 3.0',
        'Programming Language :: Python :: 3.1',
        'Programming Language :: Python :: 3.2',
        'Programming Language :: Python :: 3.3',
        'Programming Language :: Python :: 3.4',
        'Programming Language :: Python :: 3.5',
        'Programming Language :: Python :: 3.6',

I've made a PR at https://github.com/stub42/pytz/pull/3.

(I wasn't sure of the workflow: is a PR at GitHub enough, is this issue unnecessary, or do you prefer patches here? Thanks!)