Update Python version specification to use a list format in GitHub Actions workflow

Author: Vianpyro

.github/workflows/pytest.yml

@@ -10,7 +10,7 @@ jobs:
     strategy:
       matrix:
         os: [ubuntu-latest, macos-latest, windows-latest]
-        python-version: "3.x"
+        python-version: ["3.x"]
 
     steps:
     - name: Checkout code