Unpin sphinx

[larsevj]

Issue
Resolves #5893

Pre review checklist

• Read through the code changes carefully after finishing work
• Make sure tests pass locally (after every commit!)
• Prepare changes in small commits for more convenient review (optional)
• PR title captures the intent of the changes, and is fitting for release notes.
• Updated documentation
• Ensured that unit tests are added for all new behavior (See
  Ground Rules),
  and changes to existing code have good test coverage.

Pre merge checklist

• Added appropriate release note label
• Commit history is consistent and clean, in line with the contribution guidelines.

[codecov-commenter]

Codecov Report

Merging #6555 (4f9a4b6) into main (1fbcf37) will not change coverage.
Report is 1 commits behind head on main.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #6555   +/-   ##
=======================================
  Coverage   83.52%   83.52%           
=======================================
  Files         346      346           
  Lines       20786    20786           
  Branches      948      948           
=======================================
  Hits        17362    17362           
  Misses       3130     3130           
  Partials      294      294           

see 1 file with indirect coverage changes

📣 Codecov offers a browser extension for seamless coverage viewing on GitHub. Try it in Chrome or Firefox today!

[larsevj]

Have successfully built the docs locally and during the GitHub tests which was the place where the error originally occured.

[andreas-el]

LGTM