ci: add python39 for appveyor

Signed-off-by: Rui Chen <rui@meetup.com>

Author: chenrui333

.appveyor.yml

@@ -9,6 +9,10 @@ environment:
     PYTHON: 'C:\Python38\python.exe'
   - GENERATOR: 'Visual Studio 14 Win64'
     PYTHON: 'C:\Python38-x64\python.exe'
+  - GENERATOR: 'Visual Studio 14'
+    PYTHON: 'C:\Python39\python.exe'
+  - GENERATOR: 'Visual Studio 14 Win64'
+    PYTHON: 'C:\Python39-x64\python.exe'
 
 init:
   - cmd: '%PYTHON% -m pip install -U nose wheel'