Fix missing-references.json.

[anntzer]

I think what happened is that the way nested classes are referred to in
inheritance diagrams has improved (the qualified name is more correct
now), which caused missing-references.json to go out of sync.
Unfortunately this means missing-references.json can either be
compatible with sphinx<3.0 or with sphinx>3.0, but to be compatible with
both we'd likely need to generate one with both versions and merge the
two json files, which seems not worth it. Instead, just drop nitpicky
mode for older sphixes (sphinges?).

PR Summary

PR Checklist

• Has Pytest style unit tests
• Code is Flake 8 compliant
• New features are documented, with examples if plot related
• Documentation is sphinx and numpydoc compliant
• Added an entry to doc/users/next_whats_new/ if major new feature (follow instructions in README.rst there)
• Documented in doc/api/api_changes.rst if API changed in a backward-incompatible way

[lumberbot-app]

Owee, I'm MrMeeseeks, Look at me.

There seem to be a conflict, please backport manually. Here are approximate instructions:

1. Checkout backport branch and update it.

$ git checkout v3.2.x
$ git pull

2. Cherry pick the first parent branch of the this PR on top of the older branch:

$ git cherry-pick -m1 9f821e49051da54d912f5177e74b66ea37096e7d

3. You will likely have some merge/cherry-pick conflict here, fix them and commit:

$ git commit -am 'Backport PR #17045: Fix missing-references.json.'

4. Push to a named branch :

git push YOURFORK v3.2.x:auto-backport-of-pr-17045-on-v3.2.x

5. Create a PR against branch v3.2.x, I would have named this PR:

"Backport PR #17045 on branch v3.2.x"

And apply the correct labels and milestones.

Congratulation you did some good work ! Hopefully your backport PR will be tested by the continuous integration and merged soon!

If these instruction are inaccurate, feel free to suggest an improvement.

[tacaswell]

Working on backporting this, just going to re-generate the json file on the 3.2.x branch.