Add Python 3.9 for Windows (#147)

appveyor.yml

@@ -1,7 +1,7 @@
 environment:
   matrix:
     # For Python versions available on Appveyor, see
-    # http://www.appveyor.com/docs/installed-software#python
+    # https://www.appveyor.com/docs/windows-images-software/#python
     - PYTHON: "C:\\Python35"
     - PYTHON: "C:\\Python35-x64"
     - PYTHON: "C:\\Python36"
@@ -10,6 +10,8 @@ environment:
     - PYTHON: "C:\\Python37-x64"
     - PYTHON: "C:\\Python38"
     - PYTHON: "C:\\Python38-x64"
+    - PYTHON: "C:\\Python39"
+    - PYTHON: "C:\\Python39-x64"
 
 install:
   # Download and install python if we don't have the version we need