Python 3.6 invalid escape sequence deprecation fix

[scop]

https://docs.python.org/3/whatsnew/3.6.html#deprecated-python-behavior

[codecov-io]

Codecov Report

Merging #74 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master      #74   +/-   ##
=======================================
  Coverage   95.02%   95.02%           
=======================================
  Files           3        3           
  Lines         703      703           
  Branches      114      114           
=======================================
  Hits          668      668           
  Misses         20       20           
  Partials       15       15

Impacted Files	Coverage Δ
cssselect/parser.py	95.09% <100%> (ø)	✅

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 386afc5...8b3aa08. Read the comment docs.

[kmike]

A good catch, thanks!