Add ipython to docs deps so nbsphinx Pygments lexer resolves under -W

CI's new Sphinx HTML build (-W) failed with "Pygments lexer name
'ipython3' is not known [misc.highlighting_failure]" warnings on every
.ipynb in docs/tutorials/. nbsphinx renders notebook code cells with the
ipython3 lexer; that lexer ships with the IPython package and is not
included in our docs extras. My local venv had IPython transitively
(common dev dep), masking the gap. RTD has been emitting these warnings
silently since it builds without -W.

Added "ipython>=8.0" to all three sync'd surfaces:
- pyproject.toml [project.optional-dependencies.docs]
- .readthedocs.yaml post_install pip install
- .github/workflows/docs-tests.yml sphinx-build job

Verified locally with python3 -c "from pygments.lexers import
get_lexer_by_name; print(get_lexer_by_name('ipython3'))" — returns
<pygments.lexers.IPython3> when IPython is installed.

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

Author: igerber

.github/workflows/docs-tests.yml

@@ -92,7 +92,9 @@ jobs:
         # Keep in sync with pyproject.toml [project.optional-dependencies.docs]
         # and .readthedocs.yaml post_install. Skips the maturin/Rust build:
         # docs/conf.py imports diff_diff via sys.path from checked-out source.
-        run: pip install "numpy>=1.20.0" "pandas>=1.3.0" "scipy>=1.7.0" "sphinx>=6.0" "pydata-sphinx-theme>=0.15" "sphinxext-opengraph>=0.9" "sphinx-sitemap>=2.5" "nbsphinx>=0.9" "matplotlib>=3.5"
+        # ipython provides the 'ipython3' Pygments lexer that nbsphinx uses
+        # for notebook code cells; without it -W fires highlighting_failure.
+        run: pip install "numpy>=1.20.0" "pandas>=1.3.0" "scipy>=1.7.0" "sphinx>=6.0" "pydata-sphinx-theme>=0.15" "sphinxext-opengraph>=0.9" "sphinx-sitemap>=2.5" "nbsphinx>=0.9" "matplotlib>=3.5" "ipython>=8.0"
 
       - name: Build docs with warnings as errors
         # SPHINXOPTS="-W" turns every Sphinx warning into a build failure.