Read the docs build failures

[jacobtomlinson]

Looks like RTD builds started failing at some point today. Spotted the failure in #7851 but it appears unrelated to the changes. I've rerun the build and it failed again so opened #7852 with no changes and it also failed there.

The error says Unknown interpreted text role "ivar". This reference was introduced in #7586 (cc @crusaderky) but I doubt it is the problem.

/home/docs/checkouts/readthedocs.org/user_builds/distributed/envs/7851/lib/python3.11/site-packages/distributed/scheduler.py:docstring of distributed.scheduler.Scheduler.is_idle:7: ERROR: Unknown interpreted text role "ivar".

My guess is that some docs dependency had a release today that broke something. However, I haven't been able to reproduce it locally and cant see any obvious dependencies in docs/requirements.txt that got released today.