Treat Python versions as strings

They're not floats, which YAML would try to treat them as by default.

Author: PeterJCLaw

.github/workflows/python-test.yml

@@ -16,7 +16,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        python-version: [3.7, 3.8, 3.9, 3.10, 3.11]
+        python-version: ['3.7', '3.8', '3.9', '3.10', '3.11']
 
     steps:
     - uses: actions/checkout@v2