Change versions from number to string

actions/setup-python#160 (comment)

.github/workflows/flake8-pytest.yml

@@ -15,7 +15,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: [3.5, 3.6, 3.7, 3.8, 3.9, 3.10]
+        python-version: ["3.5", "3.6", "3.7", "3.8", "3.9", "3.10"]
 
     steps:
     - uses: actions/checkout@v2