Regexp change for future compatibility. Fixes #347

[mvasilkov]

Le this: #347

[codecov-io]

Codecov Report

Merging #379 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #379   +/-   ##
=======================================
  Coverage   90.76%   90.76%           
=======================================
  Files          50       50           
  Lines        6950     6950           
  Branches     1328     1328           
=======================================
  Hits         6308     6308           
  Misses        483      483           
  Partials      159      159

Impacted Files	Coverage Δ
html5lib/_inputstream.py	91.62% <100%> (ø)	⬆️

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[willkg]

I verified that this covers what was asked for in issue #347. Thank you for doing this!