CI: do not trust yaml, use strings

Author: tacaswell

.github/workflows/main.yaml

@@ -15,7 +15,7 @@ jobs:
       - uses: actions/checkout@v2
       - uses: actions/setup-python@v2
         with:
-          python-version: 3.10
+          python-version: "3.10"
       - name: Install dependencies
         run: |
           sudo apt update