Add Python version 3.10 to test.yml

Quotes have been added around the version numbers to mitigate the Python 3.1 problem.

actions/runner-images#4226 (comment)

.github/workflows/test.yml

@@ -16,7 +16,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        python-version: [3.7, 3.8, 3.9]
+        python-version: ["3.7", "3.8", "3.9", "3.10"]
 
     steps:
     - uses: actions/checkout@v2