CI: Pin an older version of Ubuntu

Python 3.6 isn't available for Ubuntu 22.04 on GHA.

Author: bmerry

.github/workflows/test.yml

@@ -4,7 +4,7 @@ on: [push]
 
 jobs:
   build:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     strategy:
       matrix:
         python-version: ["3.6", "3.7", "3.8", "3.9", "3.10", "3.11", "3.12"]