Code: Remove unsupported Python 2.6 and 3.3

[hugovk]

See #14 for docs 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 (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

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Also removed this comment because Travis now supports it:

# Expecting Coverage to fail for pypy3 until Travis supports PyPy3 5.5+ (as of May 2017)

[coveralls]

Coverage remained the same at 100.0% when pulling a9ebf05 on hugovk:rm-unsupported into d149405 on ljcooke:develop.

[ljcooke]

Thanks for this. Agreed about dropping support for these versions.