ops: Python 3.13 support

We also drop Python 3.9 because there's no SciPy version that supports
both 3.9 and 3.13. We mask system Python to force Poetry to use the
version provided by the setup-python action.

.github/workflows/ci.yml

@@ -30,12 +30,18 @@ jobs:
           - '3.10'
           - '3.11'
           - '3.12'
+          - '3.13'
 
     steps:
 
       - name: Checkout the repository
         uses: actions/[email protected]
 
+      - name: Mask existing system Python
+        run: >
+          which -a python{,3} | tr '\n' '\0' | xargs -0 stat -t &&
+          which -a python{,3} | tr '\n' '\0' | xargs -0 sudo rm -f
+
       - name: Setup Python
         uses: actions/[email protected]
         with: