Allow uppercase hex chararcters in CSS colour check

[hugovk]

Fixes #354.

[willkg]

This is awesome work! As soon as all the CI things are green, I'll merge this. Thank you!

[codecov-io]

Codecov Report

Merging #377 into master will decrease coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #377      +/-   ##
==========================================
- Coverage   90.78%   90.76%   -0.03%     
==========================================
  Files          50       50              
  Lines        6927     6950      +23     
  Branches     1322     1328       +6     
==========================================
+ Hits         6289     6308      +19     
- Misses        482      483       +1     
- Partials      156      159       +3

Impacted Files	Coverage Δ
html5lib/filters/sanitizer.py	93.16% <ø> (ø)	⬆️
html5lib/tests/test_sanitizer.py	94.59% <100%> (+0.65%)	⬆️
html5lib/treebuilders/__init__.py	81.81% <0%> (-18.19%)	⬇️

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[willkg]

Thank you!