Use Ubuntu 20.04 runner for tests

GitHub has deprecated 18.04, so we can't continue running tests on that
old distribution. Unfortunately, we can't switch to the latest 22.04
since GitHub doesn't package anything older than Python 3.7 for that
runner.

.github/workflows/test.yml

@@ -4,7 +4,7 @@ on: [push, pull_request]
 
 jobs:
   test:
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     strategy:
       matrix:
         python-version: