Fix our Python version checks and stability level in the packaging.
Review Request #9740 — Created March 4, 2018 and submitted
setup.pywas still allowing usage on Python 2.6, and
didn't have complaints about being installed on 3.x. This fixes up those
version checks to reflect the supported version range.
It also bumps the Development Status classifier to show that the package
is stable, now that we've hit 1.0.
Tested with different Python versions.