Fix YAML syntax (string not number)

.github/workflows/test.yml

@@ -6,7 +6,7 @@ jobs:
     strategy:
       matrix:
         os: [ubuntu-latest, macos-latest, windows-latest]
-        python-version: [3.6, 3.7, 3.8, 3.9, 3.10, pypy-3.7, pypy-3.8]
+        python-version: ['3.6', '3.7', '3.8', '3.9', '3.10', pypy-3.7, pypy-3.8]
         exclude:
           - os: windows-latest
             python-version: pypy-3.7