CI: build with python 3.9

Author: kba

.github/workflows/cd-pypi.yml

@@ -16,6 +16,8 @@ jobs:
       - uses: actions/checkout@v4
       - name: Set up Python
         uses: actions/setup-python@v5
+        with:
+          python-version: 3.9
       - name: Build package
         run: make build
       - name: Publish package distributions to PyPI