ci: make sure versions are strings, not floats

Author: dvarrazzo

.github/workflows/packages-pool.yml

@@ -22,7 +22,7 @@ jobs:
 
       - uses: actions/setup-python@v5
         with:
-          python-version: 3.9
+          python-version: "3.9"
 
       - name: Install the build package
         run: pip install build

.github/workflows/packages-src.yml

@@ -24,7 +24,7 @@ jobs:
 
       - uses: actions/setup-python@v5
         with:
-          python-version: 3.9
+          python-version: "3.9"
 
       - name: Install the build package
         run: pip install build