Fix GitHub workflow for testing with Python 3.10

Author: Cito

.github/workflows/test-package.yml

@@ -9,7 +9,7 @@ jobs:
     strategy:
       max-parallel: 4
       matrix:
-        python-version: [3.6, 3.7, 3.8, 3.9, 3.10, pypy3]
+        python-version: ['3.6', '3.7', '3.8', '3.9', '3.10', 'pypy3']
 
     steps:
     - uses: actions/checkout@v2