setup: add extra logic to try and recommend a new pip on bad Python
Modern pip can detect supported Python versions (which we now
declare), and pull down a reasonable release. This trick was suggested
in http://bit.ly/pycon2017-build-bridges, and seems like a good
defensive maneuver so that when we want to move to Python 3 it's
less risky for existing users.
This moves the version-check logic after defining our printf function
so we can print more informative messages.