Convert Python versions to strings in test matrix

Author: klane

.github/workflows/test.yml

@@ -24,7 +24,7 @@ jobs:
       fail-fast: false
       matrix:
         os: [Ubuntu, macOS, Windows]
-        python-version: [3.7, 3.8, 3.9, 3.10]
+        python-version: ['3.7', '3.8', '3.9', '3.10']
 
     steps:
       - name: Checkout