fix: handle backslash produced by an Unicode hex sequence

[fancyweb]

Using the Unicode hex sequence \5c produces a \ but it is not escaped. Therefore, it ends up being treated as a simple escape.

[Gallaecio]

Looks good to me, thanks!

[codecov]

Codecov Report

Merging #137 (7db25e7) into master (ddd9784) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #137      +/-   ##
==========================================
+ Coverage   95.46%   95.50%   +0.03%     
==========================================
  Files           2        3       +1     
  Lines         904      912       +8     
  Branches      161      162       +1     
==========================================
+ Hits          863      871       +8     
  Misses         24       24              
  Partials       17       17              

Impacted Files	Coverage Δ
cssselect/parser.py	96.28% <100.00%> (+0.01%)	⬆️
cssselect/__init__.py	100.00% <0.00%> (ø)

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[fancyweb]

Please don't merge yet because it doesn't fix all the cases (thanks to @MatTheCat for spotting it).
\\5c should yield \5c but would yield \\ (currently it yields \ btw).