I asked a similar question here: https://github.com/orgs/sphinx-doc/discussions/12333

Happy to close the discussion if this is a better place.

Ok, I see what happened actually. It's because the dependencies of sphinxcontrib- were not updated accordingly, namely we are installing sphinx as sphinx (so we are only using the release dependencies and not the test dependencies for sphinx).

Is there a way to tell tox to install the sphinx test dependencies in tox.ini?

PS: The argument against adding defusedxml as a runtime dependency is that, for consistency, we would also need to add pytest as a runtime dependency since we are using pytest in sphinx.testing.fixture. One could argue that because one should have pytest for using the plugin, we don't need that but technically speaking, both of them should then be present.

Now, I am willing to actually split the test dependencies into dev and test dependencies where dev dependencies would be those needed by extensions and test dependencies would only be for our tests (in particular, test would be a superset of dev, e.g., this would include cython and setuptools packages).

Now, I am willing to actually split the test dependencies into dev and test

yes I feel this is what should happen; I was about to say, your first responses seemed to be conflating sphinx internal development with the actual opened issue of users interacting with sphinx.testing.fixture 😅

(in general, it is a bit of an open question in python packaging, where/how you define internal development dependencies vs defining extra user dependencies)

I think it would also be useful to avoid importing defusedxml in global scope. It is only used by etree_parse(), and not every consumer of sphinx.testing uses this function.

For example, breathe is currently broken by this. However, it does not use this API and without the import in global scope, it doesn't have to be updated to depend on defusedxml (or sphinx[dev], as suggested above).