Add code and metadata to enforce the use of Python 3.

[petterreinholdtsen]

Tell setup.py to require python >= 3.0, and throw a RuntimeError if the sys.version_info.major is below 3.

Fixes #187.

[jnweiger]

This does not fix #187 for me -- but should be merged anyway, as enforcing python3 is a good thing. I can confirm that it does no harm and works as intended.