setup non-system python in CI's sdist build job

Author: 2bndy5

.github/workflows/python-packaging.yml

@@ -155,6 +155,9 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v4
+      - uses: actions/setup-python@v5
+        with:
+          python-version: 3.x
       - name: Build sdist
         uses: PyO3/maturin-action@v1
         with: