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. 

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23101?src=pr&el=h1) Report
> Merging [#23101](https://codecov.io/gh/pandas-dev/pandas/pull/23101?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/c8ce3d01e9ffafc24c6f9dd568cd9eb7e42c610c?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/23101/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/23101?src=pr&el=tree)

```diff
@@           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)` | :arrow_up: |
| #single | `42.3% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23101?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23101?src=pr&el=footer). Last update [c8ce3d0...c791954](https://codecov.io/gh/pandas-dev/pandas/pull/23101?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks!