Updated dependency to sphinx 7.4.0, and enabled linkcheck testing.

[MicahGale]

Description

An upstream bug with Sphinx and linkcheck failing for PDFs with anchors has been resolved. This just requires a version of Sphinx with this fix, and requires linkcheck to pass.

Fixes #414

Checklist

• I have performed a self-review of my own code
• I have made corresponding changes to the documentation (if applicable)
• I have added tests that prove my fix is effective or that my feature works (if applicable)

[MicahGale]

This is failing because pip in GHA doesn't have the latest version of Sphinx. I assume that GHA python uses a GitHub clone of pypi. The version of Sphinx I need is less than 10 hours old. I'm thinking of trying these again in 24 hours and seeing if the cache is updated daily.

[MicahGale]

The issue is actually that sphinx 7.4.0 doesn't support python 3.8. I think I will I just updated the build testing, and docs deploy to use a later version of python.

[tjlaboss]

Python 3.8 is due to reach its scheduled upstream end-of-life in October 2024.

[MicahGale]

Python 3.8 is due to reach its scheduled upstream end-of-life in October 2024.

Yep see #201. IMO: all supported versions of python will be supported for users, but only the latest version of python will be officially supported for dev. This change would move dev support from 3.8 to 3.9. I think this is fine as building docs is a "dev" task.