Add Python 3.9 to Appveyor

Signed-off-by: Gábor Lipták <gliptak@gmail.com>

Author: gliptak

appveyor.yml

@@ -1,13 +1,16 @@
 # Do not build feature branch with open Pull Requests
 skip_branch_with_pr: true
 
+image: Visual Studio 2019
+
 # environment variables
 environment:
   matrix:
     - PYTHON: "C:\\Python35-x64"
     - PYTHON: "C:\\Python36-x64"
     - PYTHON: "C:\\Python37-x64"
     - PYTHON: "C:\\Python38-x64"
+    - PYTHON: "C:\\Python39-x64"
 
 # scripts that run after cloning repository
 install: