PR: Pin Sphinx to version 5.1.1 to prevent errors on MacOS installer build

[dalthviz]

Description of Changes

• Wrote at least one-line docstrings (for any new functions)
• Added unit test(s) covering the changes (if testable)

• Included a screenshot or animation (if affecting the UI, see Licecap)

To prevent the MacOS installer build check from failing on PRs ping sphinx to be 5.1.1

Issue(s) Resolved

Fixes #19618

Affirmation

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and understand I am releasing the same under Spyder's MIT (Expat) license.

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[mrclary]

@dalthviz, well, maybe it's not py2app? the same error is occurring. I'll investigate.

[mrclary]

The only other differences I see in the build environment are:

• Successful -> setuptools=65.3.0; sphinx=5.1.1
• Unsuccessful -> setuptools=65.4.0; sphinx=5.2.0.post0

We could try pinning these instead, one-by-one, to see if any any are the culprit.

[dalthviz]

Seems like the problem was Sphinx 👍