Add python 3.9 to CI matrix

Author: kunaltyagi

.github/workflows/pytest.yml

@@ -9,7 +9,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: [3.6, 3.7, 3.8]
+        python-version: [3.6, 3.7, 3.8, 3.9]
 
     steps:
       # Checks-out your repository under $GITHUB_WORKSPACE, so your job can access it