Improve Python 2 & 3 compatiblity, declare it in setup.py

[moreati]

These are some additions for your taichino#6 PR, they:

• Ensure the README is properly decoded for long_description in Python 2.x
• Declare supported versions, for the benefit of https://python3wos.appspot.com/ etc.
• Correct a string exception, not supported in Python 3.x
• Allow a single wheel to serve all python versions
• Make print() behaviour consistent across all Python versions

[morenosan]

Thank you!