Docs: Remove unsupported Python 2.6 and 3.3

[hugovk]

See #13 for code changes.

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 (https://github.com/pypa/pip/issues/3955)
• Not much PyPI traffic (June 2016) https://github.com/pypa/pip/issues/3796

3.3

• pip 10 will deprecate Python 3.3 support, pip 11 won't support it (https://github.com/pypa/pip/issues/3796)
• Very little PyPI traffic (June 2016) https://github.com/pypa/pip/issues/3796