Executable script calls for python2

[NJDFan]

The main executable bin/rst2ansi starts with #!/usr/bin/python which aliases python2 (PEP 394). Should be #!/usr/bin/python3 or even better #!/usr/bin/env python3

[Snaipe]

The library was ported to python 2.7 in #2, so I guess we should still be using #!/usr/bin/env python like we're doing right now. Does it still work on python 2? If not, it seems sensible to update the shebang.

[NJDFan]

Nearly so; it does with a from __future__ import print_function. I put in a pull request that updates the setup.py to work correctly under 2/3. It does come at a cost of requiring setuptools rather than allowing for a clean fallback to distutils, but I feel like that's pretty much where the world is right now.