Drop python 2.6 -> 3.5 support

As stated in our release announcement Stem 2.x will discontinue Python 2.x
support now that the PSF has dropped it...

  https://blog.atagar.com/stem-release-1-8/
  https://www.python.org/doc/sunset-python-2/

Presuming our next release is a year out, Python 3.6 is the oldest non-EOL
version so picking it as our new minimum requirement...

  https://devguide.python.org/#status-of-python-branches

No doubt this compatibility change will let us make far more simplifications,
but for the moment simply addressing low hanging fruit.