FIX: change ubuntu latest to ubuntu-20.04 in test.yml

Author: amansquareboat

.github/workflows/test.yml

@@ -4,7 +4,7 @@ on: [push, pull_request]
 
 jobs:
     build:
-        runs-on: ubuntu-latest
+        runs-on: ubuntu-20.04
         strategy:
             matrix:
                 python: [3.6, 3.7, 3.8, 3.9, '3.10']