Ignore config cache warnings in sphinx

[stephprince]

Motivation

The sphinx_gallery_conf configuration contains classes that are unpickleable and will now warn the user. This warning causes the link checker workflow to fail.

How to test the behavior?

Update sphinx and run the following:

cd docs
make clean && make html

Checklist

• Did you update CHANGELOG.md with your changes?
• Does the PR clearly describe the problem and the solution?
• Have you reviewed our Contributing Guide?
• Does the PR use "Fix #XXX" notation to tell GitHub to close the relevant issue numbered XXX when the PR is merged?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.70%. Comparing base (6d0be17) to head (65cc90d).

Additional details and impacted files

@@           Coverage Diff           @@
##              dev    #1105   +/-   ##
=======================================
  Coverage   88.70%   88.70%           
=======================================
  Files          45       45           
  Lines        9745     9745           
  Branches     2767     2767           
=======================================
  Hits         8644     8644           
  Misses        779      779           
  Partials      322      322           

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