Drop support for EOL Python 2.6

[hugovk]

Latest master is failing on Python 2.6 because pytest no longer supports it.
https://travis-ci.org/hugovk/cssselect/builds/316469101

It was also suggested to drop it in #61 (comment):

+1 to drop Python 3.2 and Python 2.6 support.

Python 2.6 has been EOL for 4 years and hasn't received a security update since then. It's also little used.

Here's the pip installs for cssselect from PyPI for the last month (via pypinfo --percent --pip --markdown cssselect pyversion)

python_version	percent	download_count
2.7	63.8%	145,167
3.6	19.3%	43,801
3.5	11.3%	25,618
3.4	5.3%	12,157
3.7	0.2%	404
2.6	0.1%	250
3.3	0.1%	127
3.2	0.0%	7
None	0.0%	1

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Python 3.3 is also EOL and even less used. Should that be dropped as well?

[codecov]

Codecov Report

Merging #81 into master will increase coverage by 0.23%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #81      +/-   ##
==========================================
+ Coverage   95.02%   95.25%   +0.23%     
==========================================
  Files           3        2       -1     
  Lines         703      695       -8     
  Branches      114      113       -1     
==========================================
- Hits          668      662       -6     
+ Misses         20       19       -1     
+ Partials       15       14       -1

Impacted Files	Coverage Δ
cssselect/xpath.py	94.82% <100%> (ø)	⬆️
cssselect/parser.py	95.55% <100%> (+0.45%)	⬆️
cssselect/__init__.py

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[redapple]

Looks good to me. Thanks @hugovk !

[hugovk]

@redapple What do you think about dropping EOL Python 3.3?

[kmike]

Thanks @hugovk!

What do you think about dropping EOL Python 3.3?

I guess an argument against dropping 3.3 could be that lxml still supports Python 3.3, and cssselect is documented at http://lxml.de/cssselect.html, so we may keep 3.3 in respect. But we're dropping 2.6 (which lxml also supports), so this argument is not good :) I'm fine with dropping 3.3 as well.

[hugovk]

See PR #82 to drop 3.3.