Add Python3 classifier to setup.py

[tirkarthi]

I used caniusepython3 to test Python 3 support against my project and found that twython was reported as not compatible with Python 3 though twython's README says seamless support with Python 3. From the README of caniusepython3 adding Programming Language :: Python :: 3 classifier to setup.py will be helpful.

Thanks for the library.