[MRG+1] Drop support for EOL Python 3.3

[hugovk]

Python 3.3 is EOL and no longer receives security updates. 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

[codecov]

Codecov Report

Merging #82 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #82   +/-   ##
=======================================
  Coverage   95.25%   95.25%           
=======================================
  Files           2        2           
  Lines         695      695           
  Branches      113      113           
=======================================
  Hits          662      662           
  Misses         19       19           
  Partials       14       14

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