[ci] Dropped python 3.6, added 3.8 and 3.9

Author: nemesifier

.github/workflows/ci.yml

@@ -18,8 +18,9 @@ jobs:
       fail-fast: false
       matrix:
         python-version:
-          - 3.6
           - 3.7
+          - 3.8
+          - 3.9
 
     steps:
       - uses: actions/checkout@v2