DOC: Ignoring F821 in developer.rst, that are breaking the build
I merged #18201 without a rebase, and I didn't realize it'd break the CI, because of flake8 errors (variable not defined) in an example that is not supposed to run.

Fixing the CI by ignoring the `F821` errors.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24160?src=pr&el=h1) Report
> Merging [#24160](https://codecov.io/gh/pandas-dev/pandas/pull/24160?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/c91115172ce1e7324ff9422cb38f520496df2573?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/24160/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24160?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #24160   +/-   ##
=======================================
  Coverage    92.2%    92.2%           
=======================================
  Files         162      162           
  Lines       51700    51700           
=======================================
  Hits        47670    47670           
  Misses       4030     4030
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.6% <ø> (ø)` | :arrow_up: |
| #single | `43.02% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24160?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/24160?src=pr&el=footer). Last update [c911151...064a87e](https://codecov.io/gh/pandas-dev/pandas/pull/24160?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/24160?src=pr&el=h1) Report
> Merging [#24160](https://codecov.io/gh/pandas-dev/pandas/pull/24160?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/c91115172ce1e7324ff9422cb38f520496df2573?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/24160/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24160?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #24160   +/-   ##
=======================================
  Coverage    92.2%    92.2%           
=======================================
  Files         162      162           
  Lines       51700    51700           
=======================================
  Hits        47670    47670           
  Misses       4030     4030
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.6% <ø> (ø)` | :arrow_up: |
| #single | `43.02% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24160?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/24160?src=pr&el=footer). Last update [c911151...064a87e](https://codecov.io/gh/pandas-dev/pandas/pull/24160?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 These comments will show up in the rendered docs, right?

What's the plan for handling this long-term? Yes, I think they are rendered. For the long term we may try to either avoid having any error (in this case define `metadata`), or to ignore them in `flake8-rst`. It surely would be nice to not show them to the user.

I can take a look for better ways, but wanted to do a quick fix, as this is currently breaking the CI.