Update Sphinx to version 9

Bumping Sphinx isn't actually going to work, looks like our sphinxcontrib_trio isn't really compatible with Sphinx 9 after they rewrote autodoc. There's an option to use the old code still which would probably make it work temporarily, but we might need to redo that module. _Originally posted by @TeamSpen210 in https://github.com/python-trio/trio/issues/3363#issuecomment-3604070057_