Use modern Python syntax set literals and dict comprehension

[jdufresne]

Available since Python 2.7. Instances discovered using pyupgrade.

[codecov-io]

Codecov Report

Merging #415 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #415   +/-   ##
=======================================
  Coverage   90.79%   90.79%           
=======================================
  Files          50       50           
  Lines        6952     6952           
  Branches     1328     1328           
=======================================
  Hits         6312     6312           
  Misses        481      481           
  Partials      159      159

Impacted Files	Coverage Δ
html5lib/treebuilders/base.py	85.64% <ø> (ø)	⬆️
html5lib/tests/test_serializer.py	92.75% <100%> (ø)	⬆️
html5lib/tests/tokenizer.py	83.33% <100%> (ø)	⬆️
html5lib/constants.py	100% <100%> (ø)	⬆️
html5lib/_inputstream.py	91.92% <100%> (ø)	⬆️
html5lib/html5parser.py	98.31% <100%> (ø)	⬆️

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[hugovk]

Looks good.