Readthedocs CI failure

[williamfgc]

Describe the bug
Current readthedocs build fails on CI related to Sphinx packages with message:
Could not import extension sphinx.builders.linkcheck

To Reproduce
Steps to reproduce the behavior:
From latest CI check:
https://readthedocs.org/projects/qmcpack/builds/20387555/

Expected behavior

System:
N/A

Additional context
N/A

[prckent]

I did a quick investigation to see if I could avoid this problem by using the current versions of the sphinx packages in spack, thinking to then bump the requirements.txt versions. However, I ran into problems with an undeclared runtime dependency on sphinxcontrib.applehelp which I could not quickly get working. I need to work on other tasks for some days so I will not revisit this soon.

[prckent]

Failed attempt in #4590

Maybe see how other repos are faring. Our sphinx config is not special.

[prckent]

See readthedocs/readthedocs.org#10290

[prckent]

Fixed by #4592