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?
• [X] Does the PR clearly describe the problem and the solution?
• [X] Have you reviewed our Contributing Guide?
• [X] Does the PR use "Fix #XXX" notation to tell GitHub to close the relevant issue numbered XXX when the PR is merged?

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

```diff @@ 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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