ci: prep for 3.15t — install libhdf5-dev on pre-release Python; pin scikit-learn>=1.9.1 (#15283)

Land the non-3.15t-gated pieces of #15105 ahead of GA:
- build.yml/sphinx.yml: install libhdf5-dev only when running a pre-release
  interpreter (keras needs hdf5 there); guarded so it is a no-op on final builds.
- pyproject.toml: pin scikit-learn>=1.9.1, the first release with cp315t
  free-threaded wheels, so uv resolves a wheel instead of building from source.

The .python-version bump to 3.15t stays in #15105 as a draft until 3.15 GA.

.github/workflows/build.yml

@@ -20,6 +20,12 @@ jobs:
         with:
           python-version-file: .python-version
           allow-prereleases: true
+      # keras needs hdf5 on pre-release Python.
+      - run: |
+          if [ "$(python -c 'import sys; print(sys.version_info.releaselevel)')" != "final" ]; then
+            sudo apt-get update -qq
+            sudo apt-get install --yes libhdf5-dev
+          fi
       - run: uv sync --group=test
       - name: Run tests
         # opencv-python is gated out on 3.14t (no cp314t wheel yet), so skip the

.github/workflows/sphinx.yml

@@ -33,6 +33,12 @@ jobs:
         with:
           python-version-file: .python-version
           allow-prereleases: true
+      # keras needs hdf5 on pre-release Python.
+      - run: |
+          if [ "$(python -c 'import sys; print(sys.version_info.releaselevel)')" != "final" ]; then
+            sudo apt-get update -qq
+            sudo apt-get install --yes libhdf5-dev
+          fi
       - run: uv sync --group=docs
       - uses: actions/configure-pages@v6
       - run: uv run sphinx-build -c docs . docs/_build/html

pyproject.toml

@@ -23,7 +23,7 @@ dependencies = [
   "pandas>=2.3.3",
   "pillow>=11.3",
   "rich>=13.9.4",
-  "scikit-learn>=1.9",
+  "scikit-learn>=1.9.1",
   "scipy>=1.18.1",
   "statsmodels>=0.14.4",
   "sympy>=1.13.3",