Test on Ubuntu 20.04

Fixes #99

Author: JelleZijlstra

.github/workflows/ci.yml

@@ -20,7 +20,7 @@ jobs:
         # https://raw.githubusercontent.com/actions/python-versions/main/versions-manifest.json
         python-version: ["3.7", "3.7.1", "3.8", "3.8.0", "3.9", "3.9.0", "3.10", "3.10.0", "3.11-dev", "pypy3.9"]
 
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-2004
 
     steps:
       - uses: actions/checkout@v3