Drop support for EOL Python 2.6 and 3.3

[hugovk]

The CI fails with latest master for Python 2.6 builds which also causes PRs to fail (eg. #39).

The easiest way to fix it is to just drop support for Python 2.6. I see testtools already dropped it in
testing-cabal/testtools#198.

Other reasons for dropping 2.6

• Unsupported since 2013-10-29
• 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

(Python 3.3 also became EOL last month.)

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This change is 

[hugovk]

By the way, looks like the 2.6 failure is caused by pbr, and they've recently dropped 2.6 as well: openstack/pbr@5589801

[hugovk]

Here's pip install for fixtures from PyPI for the last month:

$ pypinfo --percent --pip fixtures pyversion
python_version percent download_count
-------------- ------- --------------
2.7              91.8%        164,392
3.5               5.2%          9,253
3.6               1.5%          2,713
3.4               1.1%          2,024
2.6               0.3%            450
3.3               0.1%            161
3.7               0.0%             10

[hugovk]

Rebased and updated to use the latest PyPy3 (the old PyPy3.3 now fails) and to also remove EOL Python 3.3.

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Here's the pip installs for fixtures from PyPI for May 2018:

python_version	percent	download_count
2.7	80.96%	71,570
3.5	12.99%	11,485
3.6	5.21%	4,607
3.4	0.65%	572
2.6	0.13%	117
3.7	0.05%	43
3.3	0.01%	10
Total		88,404

Source: pypinfo --start-date 2018-05-01 --end-date 2018-05-31 --percent --markdown fixtures pyversion