Update Python test matrix (drop EOL Pythons and add new ones)'

[jdufresne]

Python 2.6 and 3.3 are EOL. They are no longer receiving bug fixes, including for security issues. Python 2.6 went EOL on 2013-10-29 and Python 3.3 went EOL on 2017-09-29. For details, see:

https://devguide.python.org/#status-of-python-branches

Additionally, the latest pytest release dropped support for these Python versions. This change is causing CI failures for Python 2.6 and 3.3. For details on the pytest changes, see:

https://pytest.readthedocs.io/en/latest/changelog.html#pytest-3-3-0-2017-11-23

For additional discussion on why project maintainers may wish to drop Python 2.6 support, please see:

https://snarky.ca/stop-using-python-2-6/ http://www.curiousefficiency.org/posts/2015/04/stop-supporting-python26.html

Reduces testing resources.

[hugovk]

Here's the pip installs for webencodings from PyPI for last month, showing very low 2.6 and 3.3 numbers:

python_version	percent	download_count
2.7	42.1%	204,177
3.6	22.8%	110,262
3.5	20.0%	96,819
3.4	14.8%	71,670
3.7	0.2%	953
3.3	0.1%	361
2.6	0.1%	253
None	0.0%	4
2.8	0.0%	1
3.2	0.0%	1

Source: pypinfo --start-date -47 --end-date -17 --percent --pip --markdown webencodings pyversion

PS Looks like this job https://travis-ci.org/gsnedders/python-webencodings/jobs/325678092 failed due to a temporary network error. Restarting it will probably sort it.

[jdufresne]

Thanks for the review. I've modified the README as suggested. Tests are now green.

[jdufresne]

Since this PR originally went up, there have been new Python releases and now Python 3.4 is EOL. I have updated the test matrix to test all new Pythons.