chore: use ubuntu-20.04

Author: timurbazhirov

.github/workflows/cicd.yml

@@ -9,7 +9,7 @@ concurrency:
 jobs:
 
   run-tests:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     strategy:
       matrix:
         python-version: [3.6, 3.7, 3.8]