COMPAT: update for latest flake8
Hello @TomAugspurger! Thanks for submitting the PR.

 - There are no PEP8 issues in the file [`pandas/core/groupby/__init__.py`](https://github.com/TomAugspurger/pandas/blob/26160ffb3daacee1c96fb5f731680251095fba20/pandas/core/groupby/__init__.py) !

 - There are no PEP8 issues in the file [`pandas/util/testing.py`](https://github.com/TomAugspurger/pandas/blob/26160ffb3daacee1c96fb5f731680251095fba20/pandas/util/testing.py) !

 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23363?src=pr&el=h1) Report
> Merging [#23363](https://codecov.io/gh/pandas-dev/pandas/pull/23363?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/caea25a8b91d62b30eef99aa0c523448e19cd6db?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `75%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/23363/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/23363?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #23363   +/-   ##
=======================================
  Coverage   92.22%   92.22%           
=======================================
  Files         169      169           
  Lines       51261    51261           
=======================================
  Hits        47277    47277           
  Misses       3984     3984
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.66% <75%> (ø)` | :arrow_up: |
| #single | `42.23% <75%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/23363?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/23363/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `86.84% <0%> (ø)` | :arrow_up: |
| [pandas/core/groupby/\_\_init\_\_.py](https://codecov.io/gh/pandas-dev/pandas/pull/23363/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ3JvdXBieS9fX2luaXRfXy5weQ==) | `100% <100%> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23363?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/23363?src=pr&el=footer). Last update [caea25a...26160ff](https://codecov.io/gh/pandas-dev/pandas/pull/23363?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks!