Run github ci on ubuntu 20.04

That's the last version that supports 3.8.10.

Author: allerter

.github/workflows/python-app.yml

@@ -15,7 +15,7 @@ on:
 jobs:
   build-and-test:
 
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
 
     steps:
     - uses: actions/checkout@v2