Sync ipython>=8.0 into pyproject docs extras + .readthedocs.yaml

Companion to commit 51022b5 which added ipython to the workflow only.
Per the workflow comment "Keep in sync with pyproject.toml
[project.optional-dependencies.docs] and .readthedocs.yaml post_install",
add the same dep to the other two surfaces so RTD builds (which run
without -W) also stop emitting silent highlighting_failure warnings on
notebook code cells.

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

Author: igerber

.readthedocs.yaml

@@ -18,7 +18,7 @@ build:
       #
       # Keep in sync with pyproject.toml [project.dependencies]
       # and [project.optional-dependencies.docs].
-      - 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"
+      - 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"
 
 # Build documentation in the "docs/" directory with Sphinx
 sphinx:

pyproject.toml

@@ -73,6 +73,10 @@ docs = [
     "sphinx-sitemap>=2.5",
     "nbsphinx>=0.9",
     "matplotlib>=3.5",
+    # ipython provides the 'ipython3' Pygments lexer used by nbsphinx for
+    # notebook code cells; without it Sphinx emits highlighting_failure
+    # warnings on every notebook.
+    "ipython>=8.0",
 ]
 
 [project.urls]