Ignoring W504 (line break after binary operator) in linting

[datapythonista]

Based on the discussion in #23073, flake8 is not linting for W504. pep8speak does, and future versions of flake8 are likely to do so. So, I think we need to update our settings to ignore it explicitly, so pep8speaks is consistent with the CI, and updates on flake8 do not start to generate unwanted errors.

[codecov]

Codecov Report

Merging #23101 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #23101   +/-   ##
=======================================
  Coverage    92.2%    92.2%           
=======================================
  Files         169      169           
  Lines       50924    50924           
=======================================
  Hits        46952    46952           
  Misses       3972     3972

Flag	Coverage Δ
#multiple	90.62% <ø> (ø)	⬆️
#single	42.3% <ø> (ø)	⬆️

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[jreback]

thanks!