Add tests for Python 3.9

Author: TheGreatCabbage

appveyor.yml

@@ -3,6 +3,7 @@ environment:
     - PYTHON: "C:\\Python36"
     - PYTHON: "C:\\Python37"
     - PYTHON: "C:\\Python38"
+    - PYTHON: "C:\\Python39"
 
 init:
   - set PATH=%PYTHON%;%PYTHON%\Scripts;%PATH%