[BUG] Broken builds

[JimMadge]

Description

Build seem to have been broken for a few days. Example CI log.
A plugin wants sphinx>=5 but the jupyter-book version requires sphinx<5,>=4.
Not sure how the requirement for sphinx>5 was added, perhaps jupyter-book/sphinx install dependencies dynamically?

Expected behaviour

Builds run without error

Actual behaviour

Package incompatibility error when running jupyter-book build

How to reproduce