switch back to 20.04

Author: guzzijones

.github/workflows/tox.yml

@@ -12,7 +12,7 @@ on:
 
 jobs:
   tests:
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     strategy:
       matrix:
         python-version: ["3.6", "3.8"]