Add test for python3.9

Author: saibalmars

.github/workflows/build.yml

@@ -18,7 +18,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        python-version: [3.6, 3.7, 3.8]
+        python-version: [3.6, 3.7, 3.8, 3.9]
         os: [ubuntu-latest, macOS-latest]
         include:
           - os: ubuntu-latest