Test on Ubuntu 20.04 so as to still include Python 3.6.

Author: john-hen

.github/workflows/commit.yml

@@ -37,7 +37,7 @@ jobs:
         matrix:
             python: ['3.6', '3.7', '3.8', '3.9', '3.10', '3.11']
 
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
 
     steps:
       - name: Check out code.