Ignore config cache warnings in sphinx

[stephprince]

Motivation

Fix #1891.

The sphinx_gallery_conf configuration contains classes that are unpickleable and will now warn the user.

We could just ignore these warnings for now, or if we want to change the config I think we will have to wait on some serialization updates from sphinx.

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?
• Have you checked our Contributing document?
• Have you ensured the PR clearly describes the problem and the solution?
• Is your contribution compliant with our coding style? This can be checked running flake8 from the source directory.
• Have you checked to ensure that there aren't other open Pull Requests for the same change?
• Have you included the relevant issue number using "Fix #XXX" notation where XXX is the issue number? By including "Fix #XXX" you allow GitHub to close issue #XXX when the PR is merged.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.01%. Comparing base (ff1a03c) to head (82df9e2).

Additional details and impacted files

@@           Coverage Diff           @@
##              dev    #1893   +/-   ##
=======================================
  Coverage   92.01%   92.01%           
=======================================
  Files          27       27           
  Lines        2619     2619           
  Branches      685      685           
=======================================
  Hits         2410     2410           
  Misses        139      139           
  Partials       70       70           

Flag	Coverage Δ
integration	72.81% <ø> (ø)
unit	83.92% <ø> (ø)

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[rly]

Let's ignore the warning for now