Python 3.14 and Sphinx 9.1.0 build failure

[mikelync]

Describe the bug

I am using Sphinx 9.1.0.

On Python 3.12 my docs build successfully.
On Python 3.14 I receive the following logged error:

Versions
========

* Platform:         win32; (Windows-11-10.0.22631-SP0)
* Python version:   3.14.2 (CPython)
* Sphinx version:   9.1.0
* Docutils version: 0.22.4
* Jinja2 version:   3.1.6
* Pygments version: 2.19.2

Last Messages
=============

    api/package.instruments.IRS


    writing output... [ 34%]
    api/package.instruments.IndexFixedRateBond


    writing output... [ 34%]
    api/package.instruments.NDF

Loaded Extensions
=================

* sphinx.ext.mathjax (9.1.0)
* alabaster (1.0.0)
* sphinxcontrib.applehelp (2.0.0)
* sphinxcontrib.devhelp (2.0.0)
* sphinxcontrib.htmlhelp (2.1.0)
* sphinxcontrib.serializinghtml (2.0.0)
* sphinxcontrib.qthelp (2.0.0)
* sphinx.ext.autodoc (9.1.0)
* sphinx.ext.napoleon (9.1.0)
* sphinx.ext.autosummary (9.1.0)
* IPython.sphinxext.ipython_console_highlighting (unknown version)
* IPython.sphinxext.ipython_directive (unknown version)
* matplotlib.sphinxext.plot_directive (3.10.8)
* sphinx_automodapi.autodoc_enhancements (unknown version)
* sphinx.ext.graphviz (9.1.0)
* sphinx.ext.inheritance_diagram (9.1.0)
* sphinx_automodapi.automodsumm (unknown version)
* sphinx_automodapi.automodapi (unknown version)
* nbsphinx (0.9.8)
* sphinx.ext.intersphinx (9.1.0)
* sphinx.ext.extlinks (9.1.0)
* sphinxcontrib.googleanalytics (0.3)
* sphinx_tabs.tabs (unknown version)
* pydata_sphinx_theme (unknown version)

Traceback
=========

    Traceback (most recent call last):
      File "\venv314\Lib\site-packages\sphinx\cmd\build.py", line 432, in build_main
        app.build(args.force_all, args.filenames)
        ~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
      File "\venv314\Lib\site-packages\sphinx\application.py", line 442, in build
        self.builder.build_update()
        ~~~~~~~~~~~~~~~~~~~~~~~~~^^
      File "\venv314\Lib\site-packages\sphinx\builders\__init__.py", line 381, in build_update
        self.build(
        ~~~~~~~~~~^
            to_build,
            ^^^^^^^^^
        ...<2 lines>...
            method='update',
            ^^^^^^^^^^^^^^^^
        )
        ^
      File "\venv314\Lib\site-packages\sphinx\builders\__init__.py", line 460, in build
        self.write(docnames, updated_docnames, method)
        ~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
      File "\venv314\Lib\site-packages\sphinx\builders\__init__.py", line 748, in write
        self.write_documents(docnames)
        ~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^
      File "\venv314\Lib\site-packages\sphinx\builders\__init__.py", line 762, in write_documents
        self._write_serial(sorted_docnames)
        ~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^
      File "\venv314\Lib\site-packages\sphinx\builders\__init__.py", line 777, in _write_serial
        _write_docname(docname, env=self.env, builder=self, tags=self.tags)
        ~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
      File "\venv314\Lib\site-packages\sphinx\builders\__init__.py", line 890, in _write_docname
        builder.write_doc(docname, doctree)
        ~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^
      File "\venv314\Lib\site-packages\sphinx\builders\html\__init__.py", line 659, in write_doc
        doctree.walkabout(visitor)
        ~~~~~~~~~~~~~~~~~^^^^^^^^^
      File "\venv314\Lib\site-packages\docutils\nodes.py", line 232, in walkabout
        if child.walkabout(visitor):
           ~~~~~~~~~~~~~~~^^^^^^^^^
      File "\venv314\Lib\site-packages\docutils\nodes.py", line 232, in walkabout
        if child.walkabout(visitor):
           ~~~~~~~~~~~~~~~^^^^^^^^^
      File "\venv314\Lib\site-packages\docutils\nodes.py", line 232, in walkabout
        if child.walkabout(visitor):
           ~~~~~~~~~~~~~~~^^^^^^^^^
      [Previous line repeated 2 more times]
      File "\venv314\Lib\site-packages\docutils\nodes.py", line 224, in walkabout
        visitor.dispatch_visit(self)
        ~~~~~~~~~~~~~~~~~~~~~~^^^^^^
      File "\venv314\Lib\site-packages\sphinx\util\docutils.py", line 793, in dispatch_visit
        method(node)
        ~~~~~~^^^^^^
      File "\venv314\Lib\site-packages\sphinx_tabs\tabs.py", line 69, in visit
        attrs.pop("backrefs")
        ~~~~~~~~~^^^^^^^^^^^^
    KeyError: 'backrefs'

Reproduce the bug

To reproduce: my repo is private and my doc build is large, sorry I can't yet produce a MRE.

List your environment

No response

[oruebel]

I encountered what looks like the same issue here NeurodataWithoutBorders/nwb-overview#191 It looks like this may be an incompatibility between sphinx-tabs and docutils==0.22.4. Pinning to docutils==0.21.2 fixed the error for me.

[bsipocz]

The main branch should be docutils 0.22+ compatible, but indeed the newer python and sphinx versions should be added to the CI jobs to we can smoke out any remaining issues; and then tag a release.