Declare Python 3 compatibility in setup.py

Declare that pyregion is Python 3 - compatible in `setup.py` (and one minor Python 3 fix in an example).

@leejjoon Can you make a new release on PyPI?

The [existing 1.0.1 release](https://pypi.python.org/pypi/pyregion/1.0.1) isn't Python 3 and matplotlib 1.2 compatible.