Enforce flake8 E741
Enforce flake8 E741

#19496 ( Was added here )
But running `flake8 --select E741` returns nothing

Looks like this is a valid flake8 rule: ( unless im looking at the wrong docs )

https://lintlyci.github.io/Flake8Rules/
+1 Simple enough and Travis passed so I'll take @jbrockmendel 's +1 as approval to merge :-)

Thanks @alimcmaster1  Thanks! @WillAyd  # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22795?src=pr&el=h1) Report
> Merging [#22795](https://codecov.io/gh/pandas-dev/pandas/pull/22795?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/0480f4c183a95712cb8ceaf5682c5b8dd02e0f21?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/22795/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/22795?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #22795   +/-   ##
=======================================
  Coverage   92.18%   92.18%           
=======================================
  Files         169      169           
  Lines       50804    50804           
=======================================
  Hits        46833    46833           
  Misses       3971     3971
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.59% <ø> (ø)` | :arrow_up: |
| #single | `42.33% <ø> (ø)` | :arrow_up: |


------

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