Fix python implementation marker for setuptools 2.20

[paradoxxxzero]

Otherwise setup.py fail with error in html5lib setup command: Invalid environment marker: python_implementation == 'CPython'

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[hoppipolla-critic-bot]

Critic review: https://critic.hoppipolla.co.uk/r/6233

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[gsnedders]

Bah, Travis failing because of pip dropping 3.2 support. On the upside, easy to fix: we drop 3.2 support.

[gsnedders]

@paradoxxxzero Sorry for the delay; can you rebase this on master?

[paradoxxxzero]

Done sorry for the delay :)

[landscape-bot]

Code quality remained the same when pulling 5dac3ac on paradoxxxzero:master into 4205423 on html5lib:master.

[Chronial]

This breaks setup on older setuptools (this is my system setuptools):

> python setup.py sdist
error in html5lib setup command: Invalid environment marker: platform_python_implementation == 'CPython'
> python --version
Python 2.7.6
> easy_install --version
setuptools 3.3
> cat /etc/issue
Ubuntu 14.04.3 LTS \n \l

[gsnedders]

@Chronial see #231 for dealing with that; I presume in the future we'll break a relatively small number of setuptools releases.