Add matches-any pseudo-class: ':is()'

[julius383]

This pull request is aimed at trying to solve issue #67 by adding support for the Matches-Any Pseudo-class :is(). I have tried out the XPath expressions produced from examples of is() usage here and here and they seem to be correct. I'm hoping to work on adding more CSS Selectors Level 4 support as part of GSoC.

[codecov]

Codecov Report

Merging #109 (1ab9f30) into master (79c341b) will increase coverage by 0.11%.
The diff coverage is 97.61%.

❗ Current head 1ab9f30 differs from pull request most recent head 2547d6a. Consider uploading reports for the commit 2547d6a to get more accurate results

@@            Coverage Diff             @@
##           master     #109      +/-   ##
==========================================
+ Coverage   95.28%   95.39%   +0.11%     
==========================================
  Files           3        3              
  Lines         763      803      +40     
  Branches      130      139       +9     
==========================================
+ Hits          727      766      +39     
  Misses         20       20              
- Partials       16       17       +1     

Impacted Files	Coverage Δ
cssselect/xpath.py	94.77% <90.00%> (-0.20%)	⬇️
cssselect/parser.py	95.73% <100.00%> (+0.29%)	⬆️

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[Gallaecio]

Please, add tests to cover the new code. Existing tests should help you, and you can use https://codecov.io/gh/scrapy/cssselect/pull/109/diff to see which modified or added lines or code are currently not being covered by the tests.