Fix flake8 issues on v19, v20 and v21.0.rst
closes #24219
closes #24220
closes #24222
- [x] run `python make.py --single whatsnew/v0.19.0`, `python make.py --single whatsnew/v0.20.0` etc.
Related to: #24177

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24236?src=pr&el=h1) Report
> Merging [#24236](https://codecov.io/gh/pandas-dev/pandas/pull/24236?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/f6c7f6be80c83ad9f4680fc7bcd5bf9e5b426d92?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/24236/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24236?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #24236   +/-   ##
=======================================
  Coverage   92.21%   92.21%           
=======================================
  Files         162      162           
  Lines       51763    51763           
=======================================
  Hits        47733    47733           
  Misses       4030     4030
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.61% <ø> (ø)` | :arrow_up: |
| #single | `43% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24236?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/24236?src=pr&el=footer). Last update [f6c7f6b...78372e4](https://codecov.io/gh/pandas-dev/pandas/pull/24236?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/24236?src=pr&el=h1) Report
> Merging [#24236](https://codecov.io/gh/pandas-dev/pandas/pull/24236?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/9bc42f9d296cf717533051ee76130e72ea9ce557?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/24236/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24236?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #24236      +/-   ##
==========================================
+ Coverage   92.21%   92.21%   +<.01%     
==========================================
  Files         162      162              
  Lines       51768    51768              
==========================================
+ Hits        47739    47740       +1     
+ Misses       4029     4028       -1
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.61% <ø> (ø)` | :arrow_up: |
| #single | `43% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/24236?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/24236/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `87.41% <0%> (-0.1%)` | :arrow_down: |
| [pandas/io/json/json.py](https://codecov.io/gh/pandas-dev/pandas/pull/24236/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2pzb24vanNvbi5weQ==) | `93.09% <0%> (+0.47%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24236?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/24236?src=pr&el=footer). Last update [9bc42f9...aa8f5b4](https://codecov.io/gh/pandas-dev/pandas/pull/24236?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks @thoo 