Add support for Sphinx 9.0.0

[Copilot]

sphinx_rtd_theme pins Sphinx to <9, preventing installation with Sphinx 9.0.0.

ERROR: Cannot install sphinx-rtd-theme==3.0.2 and sphinx==9.0.0 because these package versions have conflicting dependencies.
    sphinx-rtd-theme 3.0.2 depends on sphinx<9 and >=6

Changes

• setup.cfg: Relax upper bound from sphinx >=6,<9 to sphinx >=6,<10
• tox.ini: Add sphinx90 test environment for Sphinx 9.0.x
• .circleci/config.yml: Add Sphinx 9.0 to CI matrix for py311, py312, py313

Existing tests pass with Sphinx 9.0.0.

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• dev.readthedocs.io
  • Triggering command: /home/REDACTED/.local/bin/sphinx-build sphinx-build -b html -Dhtml4_writer=0 docs/ /tmp/html (dns block)
• docs.readthedocs.io
  • Triggering command: /home/REDACTED/.local/bin/sphinx-build sphinx-build -b html -Dhtml4_writer=0 docs/ /tmp/html (dns block)
• www.sphinx-doc.org
  • Triggering command: /home/REDACTED/.local/bin/sphinx-build sphinx-build -b html -Dhtml4_writer=0 docs/ /tmp/html (dns block)

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Original prompt

---

This section details on the original issue you should resolve

<issue_title>Add support for Sphinx 9.0.0</issue_title>
<issue_description>### Problem

When trying to use sphinx_rtd_theme==3.0.2 with Sphinx==9.0.0, pip refuses to resolve dependencies because the theme currently pins Sphinx to <9:

sphinx-rtd-theme 3.0.2 depends on sphinx<9 and >=6
The user requested sphinx==9.0.0

Since Sphinx 9.0.0 is now released and installable, this strict upper bound prevents using sphinx_rtd_theme with the latest Sphinx, even for simple projects that otherwise work fine.

I’m filing this to request:

1. Support for Sphinx 9.0.0 in sphinx_rtd_theme, and
2. A relaxed upper bound in install_requires (e.g. sphinx<10) once compatibility is confirmed.

Reproducible Project

This is a pure dependency-resolution issue and does not depend on any particular RST content. A minimal repro is:

python3 -m venv venv
source venv/bin/activate

python3 -m pip install --upgrade pip

python3 -m pip install \
  "sphinx==9.0.0" \
  "sphinx_rtd_theme==3.0.2" \
  "nbsphinx"

Error Logs/Results

Relevant excerpt from pip output:

Requirement already satisfied: sphinx==9.0.0 in ./venv/lib/python3.13/site-packages (9.0.0)
Collecting sphinx_rtd_theme==3.0.2
  Using cached sphinx_rtd_theme-3.0.2-py2.py3-none-any.whl.metadata (4.4 kB)
INFO: pip is looking at multiple versions of sphinx-rtd-theme to determine which version is compatible with other requirements. This could take a while.
Collecting sphinx==9.0.0
  Using cached sphinx-9.0.0-py3-none-any.whl.metadata (5.8 kB)
ERROR: Cannot install sphinx-rtd-theme==3.0.2 and sphinx==9.0.0 because these package versions have conflicting dependencies.

The conflict is caused by:
    The user requested sphinx==9.0.0
    sphinx-rtd-theme 3.0.2 depends on sphinx<9 and >=6

ERROR: ResolutionImpossible: for help visit https://pip.pypa.io/en/latest/topics/dependency-resolution/#dealing-with-dependency-conflicts

Expected Results

Once sphinx_rtd_theme is compatible with Sphinx 9, I would expect:

• pip install "sphinx==9.0.0" "sphinx_rtd_theme==<new-version>" to succeed without dependency conflicts.
• The theme to work as usual under Sphinx 9.0.0 (basic builds, HTML output, etc.).

Environment Info

• Python Version: 3.13.9
• Sphinx Version: 9.0.0
• RTD Theme Version: 3.0.2
  </issue_description>

<agent_instructions>We need to add support for Sphinx 9.0.0 in our test cases and update the setup.cfg to accept Sphinx 9.x</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes Add support for Sphinx 9.0.0 #1665

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[humitos]

Looks good to me

[humitos]

I pulled this PR and installed it from the source. It worked fine. I was able to build the documentation with Sphinx 9.x and it worked.

[agjohnson]

This is not the right place to document the version. This document is describing major development releases, not minor releases. We've been updating the changelog page with versions and future versions/RCs.

[agjohnson]

Also, I feel we should bump the version to an RC so users installing this package via Git have the latest RC at hand. We don't need to release to PyPI yet, but that's the next step

[humitos]

This is not the right place to document the version. This document is describing major development releases, not minor releases.

We have 1.1.0 in the section. That's why I added it here.

https://sphinx-rtd-theme.readthedocs.io/en/stable/development.html#roadmap-release-1-1-0

[humitos]

Also, I feel we should bump the version to an RC so users installing this package via Git have the latest RC at hand. We don't need to release to PyPI yet, but that's the next step

OK. I pushed 9f6556b with changelog and rc version.