fix: Disallow Sphinx v5.1.0 to avoid 'exception: pop from an empty deque'

[matthewfeickert]

Description

• Disallow Sphinx v5.1.0 as it introduces an deque error that can be caused by sphinx.ext.napoleon:

Extension error (sphinx.ext.napoleon):
Handler <function _process_docstring at 0x7f43e8361a20> for event 'autodoc-process-docstring' threw an exception (exception: pop from an empty deque)

• This was fixed rapidly in Fix ValueError popping out in sphinx.ext.napoleon. sphinx-doc/sphinx#10709 and
  so will not be a problem in Sphinx v5.1.1+. (Thanks again to the amazing Sphinx team for the super rapid work they do on patch releases!)

Checklist Before Requesting Reviewer

• Tests are passing
• "WIP" removed from the title of the pull request
• Selected an Assignee for the PR to be responsible for the log summary

Before Merging

For the PR Assignees:

• Summarize commit messages into a comprehensive review of the PR

* Disallow Sphinx v5.1.0 as it introduces an deque error that can be
  caused by sphinx.ext.napoleon:
> Extension error (sphinx.ext.napoleon):
> Handler <function _process_docstring at 0x7f43e8361a20> for event 'autodoc-process-docstring'
> threw an exception (exception: pop from an empty deque)
   - This was fixed rapidly in https://github.com/sphinx-doc/sphinx/pull/10709 and
     so will not be a problem in Sphinx v5.1.1+.

[matthewfeickert]

Other CI is failing for known reasons (Issue #1918) and so this is only going to fix the docs CI.

[matthewfeickert]

As this is only to get the docs CI passing, I'm going to approve and merge this myself. As always, PRs approved by a single core dev can be reverted as needed by the rest of the dev team.