ci: build sdist on Python 3.14

The sdist build doesn't compile C (cibuildwheel handles that with its own
Pythons), and the wheel is cp310-abi3, so the choice of Python here is just
tooling. 3.14 is the latest stable.

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

Author: Peter Stenger

.github/workflows/publish-pypi.yml

@@ -20,7 +20,7 @@ jobs:
       - uses: actions/checkout@v6
       - uses: actions/setup-python@v6
         with:
-          python-version: "3.11"
+          python-version: "3.14"
       - run: pip install --upgrade build
       - run: python -m build --sdist
       - uses: actions/upload-artifact@v7