MRG: Fix documentation build

[hoechenberger]

The CircleCI image we used to use is not available anymore, see https://discuss.circleci.com/t/linux-image-deprecations-and-eol-for-2024/50177

Merge checklist

Maintainer, please confirm the following before merging.
If applicable:

• All comments are resolved
• This is not your own PR
• All CIs are happy
• PR title starts with [MRG]
• whats_new.rst is updated
• New contributors have been added to CITATION.cff
• PR description includes phrase "closes <#issue-number>"

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.54%. Comparing base (87eea28) to head (86c703a).
Report is 8 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1233      +/-   ##
==========================================
- Coverage   97.61%   97.54%   -0.08%     
==========================================
  Files          40       40              
  Lines        8685     8663      -22     
==========================================
- Hits         8478     8450      -28     
- Misses        207      213       +6     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[sappelhoff]

LGTM! Thanks Richard :)