Update setup.py to make Python 3 requirement harder to miss

[edmorley]

For example:

• adding a suitable value for python_requires (though only read by pip 9+) or else a runtime check (see here for example implementations)
• removing the Python :: 2.* trove classifiers

This would help avoid #8, #21 etc.

[sigmavirus24]

I would merge this if you sent a PR with this in it.