Quote Python version 3.9 in GitHub Actions

Author: adamchainz

.github/workflows/main.yml

@@ -21,7 +21,7 @@ jobs:
       fail-fast: false
       matrix:
         python-version:
-        - 3.9
+        - '3.9'
         - '3.10'
         - '3.11'
         - '3.12'