Start testing against Python 3.9.

Author: Fizzadar

.github/workflows/test.yml

@@ -12,7 +12,7 @@ jobs:
     strategy:
       matrix:
         os: [ubuntu-18.04]
-        python-version: [2.7, 3.6, 3.7, 3.8]
+        python-version: [2.7, 3.6, 3.7, 3.8, 3.9]
         include:
           - os: macos-10.15
             python-version: 3.8