Pinned pycodestyle
Fixes issues with the LINT failures as of recent. Quick fix with more permanent solutions split between #25697 and #25622

@TomAugspurger 
With the latest flake8 (3.7.7) and pycodestyle=2.4.0, I get an exception

```
module 'pycodestyle' has no attribute 'maximum_doc_length
```

I'm having trouble finding which versions of flake8 are compatible with which versions of pycodestyle.

Hopefully conda is smart enough to solve these properly. Ok, for reference here's what conda found

```
flake8                    3.6.0                    py37_0
pycodestyle               2.4.0                    py37_0
pyflakes                  2.0.0                    py37_0
```

and that works well. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25701?src=pr&el=h1) Report
> Merging [#25701](https://codecov.io/gh/pandas-dev/pandas/pull/25701?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/479f821ef9881c545a73878571024d6c0671f382?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/25701/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/25701?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #25701   +/-   ##
=======================================
  Coverage   91.28%   91.28%           
=======================================
  Files         173      173           
  Lines       52967    52967           
=======================================
  Hits        48350    48350           
  Misses       4617     4617
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.86% <ø> (ø)` | :arrow_up: |
| #single | `41.74% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25701?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/25701?src=pr&el=footer). Last update [479f821...449a0e7](https://codecov.io/gh/pandas-dev/pandas/pull/25701?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25701?src=pr&el=h1) Report
> Merging [#25701](https://codecov.io/gh/pandas-dev/pandas/pull/25701?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/479f821ef9881c545a73878571024d6c0671f382?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/25701/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/25701?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #25701   +/-   ##
=======================================
  Coverage   91.28%   91.28%           
=======================================
  Files         173      173           
  Lines       52967    52967           
=======================================
  Hits        48350    48350           
  Misses       4617     4617
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.86% <ø> (ø)` | :arrow_up: |
| #single | `41.74% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25701?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/25701?src=pr&el=footer). Last update [479f821...449a0e7](https://codecov.io/gh/pandas-dev/pandas/pull/25701?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @TomAugspurger all green here. Would we need to back port this as well? thanks @WillAyd 

yeah this needs a backport too Owee, I'm MrMeeseeks, Look at me.

There seem to be a conflict, please backport manually. Here are approximate instructions:

1. Checkout backport branch and update it.

```
$ git checkout 0.24.x
$ git pull
```

2. Cherry pick the first parent branch of the this PR on top of the older branch:
```
$ git cherry-pick -m1 c57d162139c629a5e90bec6c1f336606755a0f7e
```

3. You will likely have some merge/cherry-pick conflict here, fix them and commit:

```
$ git commit -am 'Backport PR #25701: Pinned pycodestyle'
```

4. Push to a named branch :

```
git push YOURFORK 0.24.x:auto-backport-of-pr-25701-on-0.24.x
```

5. Create a PR against branch 0.24.x, I would have named this PR:

> "Backport PR #25701 on branch 0.24.x"

And apply the correct labels and milestones.

Congratulation you did some good work ! Hopefully your backport PR will be tested by the continuous integration and merged soon!

If these instruction are inaccurate, feel free to [suggest an improvement](https://github.com/MeeseeksBox/MeeseeksDev).
                 we *may* need to patch https://github.com/macpython/pandas-wheels as well