Pin sphinx <7.0, docutils <0.18

[lilyminium]

Fixes #4177, #4194

Changes made in this Pull Request:

• pinned sphinx <7.0
• pinned docutils <0.18

PR Checklist

• Tests?
• Docs?
• CHANGELOG updated?
• Issue raised/referenced?

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📚 Documentation preview 📚: https://mdanalysis--4196.org.readthedocs.build/en/4196/

[github-actions]

Linter Bot Results:

Hi @lilyminium! Thanks for making this PR. We linted your code and found the following:

There are currently no issues detected! 🎉

[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (2e39563) 93.61% compared to head (798db7b) 93.61%.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #4196   +/-   ##
========================================
  Coverage    93.61%   93.61%           
========================================
  Files          193      193           
  Lines        25170    25170           
  Branches      4059     4059           
========================================
  Hits         23562    23562           
  Misses        1092     1092           
  Partials       516      516           

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[RMeli]

I run into this too recently (RMeli/spyrmsd#77); the RTD theme is not compatible with sphinx>=7: readthedocs/sphinx_rtd_theme#1463.

[lilyminium]

Thanks @RMeli! Yeah I was surprised that RTD wasn't also failing with that error. Turns out it seems to defualt to sphinx 5.0.2 anyway: https://readthedocs.org/api/v2/build/21298747.txt and https://readthedocs.org/api/v2/build/21305166.txt. It's still suitably messed
up: https://mdanalysis--4196.org.readthedocs.build/en/4196/documentation_pages/core/topologyattrs.html

I think I might have tracked it down to docutils updating to 0.18.1!

[IAlibay]

The one question but otherwise I'll approve.