Fix running tests with Python 3.10

https://github.com/actions/setup-python/issues/249#issuecomment-934299359

.github/workflows/testing.yml

@@ -18,7 +18,7 @@ jobs:
     strategy:
       matrix:
         os: [ubuntu-latest]
-        python-version: [3.6, 3.7, 3.8, 3.9, 3.10]
+        python-version: ["3.6", "3.7", "3.8", "3.9", "3.10"]
 
     steps:
     - uses: actions/checkout@v2