Update flake8 to the latest version and fix all errors

[jdufresne]

No description provided.

[codecov-io]

Codecov Report

Merging #428 into master will increase coverage by 0.05%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master     #428      +/-   ##
==========================================
+ Coverage   90.52%   90.57%   +0.05%     
==========================================
  Files          50       50              
  Lines        6973     6968       -5     
  Branches     1328     1328              
==========================================
- Hits         6312     6311       -1     
+ Misses        502      498       -4     
  Partials      159      159

Impacted Files	Coverage Δ
html5lib/tests/tokenizertotree.py	0% <ø> (ø)	⬆️
html5lib/tests/support.py	92.24% <ø> (ø)	⬆️
html5lib/treewalkers/etree.py	86.07% <ø> (ø)	⬆️
html5lib/tests/tree_construction.py	93.43% <ø> (ø)	⬆️
html5lib/tests/tokenizer.py	83.33% <ø> (ø)	⬆️
html5lib/_ihatexml.py	45.27% <ø> (ø)	⬆️
html5lib/_utils.py	84.37% <0%> (ø)	⬆️
html5lib/_inputstream.py	91.92% <100%> (ø)	⬆️
html5lib/html5parser.py	97.24% <100%> (+0.24%)	⬆️

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[gsnedders]

FWIW, I have a preference in favour of pinning flake8 and its dependencies to stop tests from starting failing without changes in html5lib, though preferably in combination with adding some bot to update pinned dependencies.

[jdufresne]

No problem. I've changed the PR to keep flake8 pinned, but now to the latest version. All errors are still fixed.

[hugovk]

preferably in combination with adding some bot to update pinned dependencies.

The two bots I know about: dependabot, now part of GitHub; and pyupbot, which I hear has been a bit flaky of late.

[gsnedders]

Yeah, we use pyupbot on web-platform-tests and it seems to have been going downhill…