using ubuntu-20.04 for runner as python 3.6 not supported with new latest ubuntu runner

Author: maxkahan

.github/workflows/ot.yml

@@ -5,10 +5,10 @@ on: [push]
 jobs:
   test:
 
-    runs-on: "ubuntu-latest"
+    runs-on: "ubuntu-20.04"
     strategy:
       matrix:
-        python-version: ["3.6", "3.7", "3.8.8", "3.9"]
+        python-version: [3.6, 3.7, 3.8, 3.9]
 
     steps:
     - uses: actions/checkout@v3