Appveyor add Python 3.9

Author: jdavid

appveyor.yml

@@ -1,5 +1,5 @@
 version: 1.3.{build}
-image: Visual Studio 2015
+image: Visual Studio 2019
 configuration: Release
 environment:
   global:
@@ -19,6 +19,8 @@ environment:
     PYTHON: 'C:\Python38\python.exe'
   - GENERATOR: 'Visual Studio 14 Win64'
     PYTHON: 'C:\Python38-x64\python.exe'
+  - GENERATOR: 'Visual Studio 14 Win64'
+    PYTHON: 'C:\Python39-x64\python.exe'
 
 matrix:
   fast_finish: true