DOC: Repair doc/source/merging.rst flake8 issues
Repairs flake8 issues, removes exclusion from ``setup.cfg``.

- [x] closes #24179 
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24185?src=pr&el=h1) Report
> Merging [#24185](https://codecov.io/gh/pandas-dev/pandas/pull/24185?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/fb7dbe016230463c93ff08fb1fc934e8c684400b?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/24185/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24185?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #24185   +/-   ##
=======================================
  Coverage   92.21%   92.21%           
=======================================
  Files         162      162           
  Lines       51723    51723           
=======================================
  Hits        47694    47694           
  Misses       4029     4029
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.61% <ø> (ø)` | :arrow_up: |
| #single | `43.03% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24185?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/24185?src=pr&el=footer). Last update [fb7dbe0...077276e](https://codecov.io/gh/pandas-dev/pandas/pull/24185?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/24185?src=pr&el=h1) Report
> Merging [#24185](https://codecov.io/gh/pandas-dev/pandas/pull/24185?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/fb7dbe016230463c93ff08fb1fc934e8c684400b?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/24185/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24185?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #24185   +/-   ##
=======================================
  Coverage   92.21%   92.21%           
=======================================
  Files         162      162           
  Lines       51723    51723           
=======================================
  Hits        47694    47694           
  Misses       4029     4029
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.61% <ø> (ø)` | :arrow_up: |
| #single | `43.03% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24185?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/24185?src=pr&el=footer). Last update [fb7dbe0...077276e](https://codecov.io/gh/pandas-dev/pandas/pull/24185?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks