Drop unsupported Python 2.6 and 3.2

[hugovk]

For #337. Includes PR #338.

Reasons for dropping Python old versions

They're all EOL. Reduces developer burden. Frees up slots in the CI.

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

2.6

• https://en.wikipedia.org/wiki/CPython#Version_history
• https://snarky.ca/stop-using-python-2-6/
• http://www.curiousefficiency.org/posts/2015/04/stop-supporting-python26.html
• http://www.python3statement.org
• 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

3.2

• pip 8 no longer supports Python 3.2 (link)
• Coverage 4 no longer supports Python 3.2 (link)
• Virtually no PyPI traffic (June 2016) Deprecate/Drop Support for Python 3.3? pypa/pip#3796

[coveralls]

Coverage increased (+1.9%) to 87.538% when pulling 299f899 on hugovk:rm-python-2.6-3.2 into 26c1d12 on nose-devs:master.