Use the standard Ubuntu runner for tests

GitHub has deprecated 18.04, so we can't continue running tests on that
old distribution.

Let's stick with whatever GitHub thinks is the current version so we
don't end up with this problem again.

.github/workflows/test.yml

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