Ignoring W504 (line break after binary operator) in linting
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. 
