Set version to Ubuntu 20.04 to allow Python 3.6. test.

.github/workflows/test.yml

@@ -2,7 +2,7 @@ name: Tests
 on: [push, pull_request]
 jobs:
   pytest:
-    runs-on: ubuntu-22.04
+    runs-on: ubuntu-20.04
     strategy:
       matrix:
         python: ["3.6", "3.7", "3.8", "3.9"]