Unpin sphinx in requirements.txt

[stevepiercy]

Sphinx is currently pinned to sphinx<9 as a temporary workaround to #2036.

Ultimately, I need to complete work on Plone Sphinx Theme v2.0.0 and release it to remove the dependency on Sphinx Book Theme.

[Shyam-Raghuwanshi]

Hi @stevepiercy we’ve implemented a temporary workaround for the original issue #2036. Could you please share your thoughts on what might be causing the underlying issue?

[stevepiercy]

@Shyam-Raghuwanshi see the issue description.