Removed Python 3.5 and added Python 3.9 in Github Action (#831)

* Removed python3.5 from CI

* Add python 3.9

.github/workflows/pythonapp.yml

@@ -8,7 +8,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: [2.7, 3.5, 3.6, 3.7, 3.8]
+        python-version: [2.7, 3.6, 3.7, 3.8, 3.9]
 
     steps:
     - uses: actions/checkout@v2