Make the package support py3

[rickmak]

Reported by pylama: https://travis-ci.org/oursky/norecaptcha/jobs/49562553

norecaptcha/captcha.py:70:1: E0602 undefined name 'unicode' [pyflakes]

[Zeioth]

I just made a fork of your project for Python3:
https://github.com/Zeioth/norecaptcha-python3

[rickmak]

What modification made? Anythings other than the unicode one?

[Zeioth]

A couple of libraries were differently organized in python 3.
Porting guide: http://docs.pythonsprints.com/python3_porting/py-porting.html
Complete diff: https://www.diffchecker.com/dlm7xsnn