Avoid the YAML parsing mistake

3.10 is parsed as a number and it is trimmed to 3.1.
That is expected behavior for numbers, but it's crazy for versions.

.github/workflows/main.yml

@@ -16,7 +16,7 @@ jobs:
       fail-fast: false
       matrix:
         # Current ubuntu-latest (Ubuntu 22.04) provides the following versions of Python:
-        python-version: [3.7, 3.8, 3.9, 3.10, 3.11]
+        python-version: ['3.7', '3.8', '3.9', '3.10', '3.11']
 
     steps: