Fix missing python 3.6 in action

Author: dannyp11

.github/workflows/test.yml

@@ -12,7 +12,7 @@ on:
 
 jobs:
   build:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     strategy:
       matrix:
         python: [3.6, 3.7, 3.8, 3.9]