Represent version number as a string

Author: wch

.github/workflows/pytest.yaml

@@ -7,7 +7,8 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: [3.7, 3.8, 3.9, 3.10]
+        # "3.10" must be a string; otherwise it is interpreted as 3.1.
+        python-version: [3.7, 3.8, 3.9, "3.10"]
       fail-fast: false
 
     steps: