ci: add Python 3.9 docs-deps install smoke job

The existing sphinx-build job runs on Python 3.11 to mirror RTD. This
new job runs the same pip install line on Python 3.9 to validate the
docs dependency floor against the project's declared minimum Python
(pyproject.toml requires-python>=3.9).

Catches drift where a docs dep upgrade silently requires a newer Python
than the project advertises - exactly the constraint that drove the
previous commit's choice of pydata-sphinx-theme>=0.16.1 (vs 0.17.1,
which requires Python>=3.10).

The job only installs deps. Full Sphinx build remains exclusively on
the 3.11 sphinx-build job; this is a fast install-only smoke (~30s).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: igerber

.github/workflows/docs-tests.yml

@@ -101,3 +101,34 @@ jobs:
         # PR #410 brought make html to 0 warnings; -W keeps it that way by
         # blocking any new warning from sneaking in.
         run: make -C docs html SPHINXOPTS="-W"
+
+  docs-deps-py39-smoke:
+    name: Validate docs deps install on Python 3.9 (floor compat)
+    if: >-
+      github.event_name != 'pull_request'
+      || contains(github.event.pull_request.labels.*.name, 'ready-for-ci')
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v6
+
+      - name: Set up Python 3.9
+        uses: actions/setup-python@v6
+        with:
+          # 3.9 is the project's declared floor (pyproject.toml requires-python
+          # >=3.9). The sphinx-build job above runs on 3.11; this job verifies
+          # the docs dependency set actually installs on the floor Python so
+          # we catch drift like pydata-sphinx-theme>=0.17.1 (which requires
+          # Python>=3.10) before it lands.
+          python-version: '3.9'
+
+      - name: Install docs dependencies
+        # Same pip install line as sphinx-build above. Just installs - does
+        # not run a Sphinx build (sphinx-build covers full rendering on 3.11).
+        # Failure here means the docs floor declared in pyproject.toml /
+        # .readthedocs.yaml / above is not installable on Python 3.9.
+        run: pip install "numpy>=1.20.0" "pandas>=1.3.0" "scipy>=1.7.0" "sphinx>=6.0" "pydata-sphinx-theme>=0.16.1" "sphinxext-opengraph>=0.9" "sphinx-sitemap>=2.5" "nbsphinx>=0.9" "matplotlib>=3.5" "ipython>=8.0"
+
+      - name: Confirm pydata-sphinx-theme version
+        # Surface the resolved version in the log for future debugging.
+        run: pip show pydata-sphinx-theme | head -3