[py-ci.yml] use quotes around Python versions

Author: chrissimpkins

.github/workflows/py-ci.yml

@@ -8,7 +8,7 @@ jobs:
     strategy:
       matrix:
         os: [ubuntu-latest, macos-latest, windows-latest]
-        python-version: [3.7, 3.8, 3.9, 3.10]
+        python-version: ["3.7", "3.8", "3.9", "3.10"]
 
     steps:
       - uses: actions/checkout@v2