Add Python 3.6, drop unsupported 2.6, 3.3

[hugovk]

Reasons for dropping old ones

• https://en.wikipedia.org/wiki/CPython#Version_history

2.6

• https://snarky.ca/stop-using-python-2-6/
• http://www.curiousefficiency.org/posts/2015/04/stop-supporting-python26.html
• Current pip 9 deprecates Python 2.6 support, pip 10 won't support it (Drop support for Python 2.6? pypa/pip#3955)
• Not much PyPI traffic (June 2016) Deprecate/Drop Support for Python 3.3? pypa/pip#3796

3.3

• pip 10 will deprecate Python 3.3 support, pip 11 won't support it (Deprecate/Drop Support for Python 3.3? pypa/pip#3796)
• Very little PyPI traffic (June 2016) Deprecate/Drop Support for Python 3.3? pypa/pip#3796

[coveralls]

Coverage remained the same at 72.222% when pulling 2d2fbaf on hugovk:update-versions into 1b096f0 on Bernhard10:master.