Added flake8 to DEV requirements
In advanced of the doc sprint this Saturday it looks like some users were confused by `git diff upstream/master -u -- "*.py" | flake8 --diff` failing even though they though they installed all development dependencies. Updating the .yml file to include this

i think you need to run the ``scripts/convert_deps.y`` and check in the changes # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20063?src=pr&el=h1) Report
> Merging [#20063](https://codecov.io/gh/pandas-dev/pandas/pull/20063?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/b6691127523f965003dbf877a358c81af5012989?src=pr&el=desc) will **increase** coverage by `0.02%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/20063/graphs/tree.svg?token=eZ4WkYLtcO&width=650&src=pr&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/20063?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #20063      +/-   ##
==========================================
+ Coverage   91.69%   91.71%   +0.02%     
==========================================
  Files         150      150              
  Lines       49112    49112              
==========================================
+ Hits        45035    45045      +10     
+ Misses       4077     4067      -10
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.1% <ø> (+0.02%)` | :arrow_up: |
| #single | `41.86% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/20063?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/20063/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `83.64% <0%> (-0.21%)` | :arrow_down: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/20063/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `66.81% <0%> (+1.73%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20063?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/20063?src=pr&el=footer). Last update [b669112...d46f2c5](https://codecov.io/gh/pandas-dev/pandas/pull/20063?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks!