ci: put python version between quote marks because YAML

Author: Exirel

.github/workflows/ci.yml

@@ -20,10 +20,10 @@ jobs:
     strategy:
       matrix:
         python-version:
-          - 3.7
-          - 3.8
-          - 3.9
-          - 3.10
+          - "3.7"
+          - "3.8"
+          - "3.9"
+          - "3.10"
     steps:
       - uses: actions/checkout@v3
       - name: Set up Python ${{ matrix.python-version }}