install python

Author: phw

.github/workflows/build.yaml

@@ -44,6 +44,11 @@ jobs:
       # IMPORTANT: this permission is mandatory for Trusted Publishing
       id-token: write
     steps:
+    - name: Set up Python ${{ matrix.python-version }}
+      uses: actions/setup-python@v5
+      with:
+        python-version: 3.9
+        cache: 'pip'
     - name: Build
       run: python -m build --sdist .
     - name: Publish package distributions to PyPI