DOC: Ignoring F821 in developer.rst, that are breaking the build

[datapythonista]

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]

Codecov Report

Merging #24160 into master will not change coverage.
The diff coverage is n/a.

@@           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)	⬆️
#single	43.02% <ø> (ø)	⬆️

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[codecov]

Codecov Report

Merging #24160 into master will not change coverage.
The diff coverage is n/a.

@@           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)	⬆️
#single	43.02% <ø> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update c911151...064a87e. Read the comment docs.

[TomAugspurger]

These comments will show up in the rendered docs, right?

What's the plan for handling this long-term?

[datapythonista]

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.