Downgrade ubuntu version to support python 3.6

Author: t-persson

.github/workflows/main.yml

@@ -10,7 +10,7 @@ on: [push, pull_request]
 jobs:
   run-tox:
     # The type of runner that the job will run on
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
 
     # Steps represent a sequence of tasks that will be executed as part of the job
     steps: