Update pytest

[gsnedders]

Fixes #411. See also #414, #429.

[codecov-commenter]

Codecov Report

Merging #497 into master will increase coverage by 0.03%.
The diff coverage is 91.66%.

@@            Coverage Diff             @@
##           master     #497      +/-   ##
==========================================
+ Coverage   90.84%   90.87%   +0.03%     
==========================================
  Files          50       50              
  Lines        6966     6969       +3     
  Branches     1326     1325       -1     
==========================================
+ Hits         6328     6333       +5     
  Misses        478      478              
+ Partials      160      158       -2     

Impacted Files	Coverage Δ
html5lib/tests/test_treewalkers.py	97.18% <87.50%> (+0.04%)	⬆️
html5lib/tests/test_serializer.py	93.10% <88.23%> (+0.04%)	⬆️
html5lib/tests/test_sanitizer.py	94.73% <92.85%> (+0.14%)	⬆️
html5lib/tests/test_encoding.py	100.00% <100.00%> (ø)
html5lib/tests/tree_construction.py	95.55% <100.00%> (+1.39%)	⬆️

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