Docs CI broken: sphinx-multiversion incompatible with Sphinx 9

[chengzhuzhang]

The publish-docs job fails on every push to main: https://github.com/E3SM-Project/zppy-interfaces/actions/runs/31208671038

File ".../sphinx_multiversion/main.py", line 88, in load_sphinx_config
TypeError: Config.read() takes 2 positional arguments but 3 were given

sphinx-multiversion 0.2.4 calls Config.read(confdir, confoverrides, add_defaults=True). Sphinx 8.1 deprecated those extra arguments and Sphinx 9.0 removed them.

sphinx is unpinned in conda/dev.yml and pyproject.toml, and CI solves the environment fresh, so this appeared on its own:

Run	Date	Sphinx	Result
28465707237	2026-06-30	8.2.3	pass
31208671038	2026-08-07	9.1.0	fail

Unrelated to the PR that happened to trigger the failing run (#53) — any push to main fails the same way. publish-docs only runs on push/workflow_dispatch (build_workflow.yml:114), so it is never exercised on PR branches.

Short-term fix

Pin sphinx<9 in both conda/dev.yml and the docs extra in pyproject.toml.

Longer term

sphinx-multiversion has had no release since 2021, so the pin only holds until something else needs Sphinx 9+. Migrating to a maintained multi-version builder (e.g. sphinx-polyversion) would be the real fix.

[chengzhuzhang]

@forsyth2 it looks like zppy has a same issue when merging just .cfg update to it's main...https://github.com/E3SM-Project/zppy/actions/runs/31209134025

[forsyth2]

I ran into this after merging E3SM-Project/zppy#774 as well. It looks like E3SM-Project/e3sm_diags#1076 is the equivalent PR to use as a model. I will update both zppy & zppy-interfaces.