[appveyor] Add Python 3.9 tests

Author: mpsonntag

appveyor.yml

@@ -13,6 +13,10 @@ environment:
     - PYTHON: "C:\\Python38"
       PYVER: 3
       BITS: 32
+    - PYTHON: "C:\\Python39"
+      APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2019
+      PYVER: 3
+      BITS: 32
     - PYTHON: "C:\\Python36-x64"
       PYVER: 3
       BITS: 64
@@ -22,6 +26,10 @@ environment:
     - PYTHON: "C:\\Python38-x64"
       PYVER: 3
       BITS: 64
+    - PYTHON: "C:\\Python39-x64"
+      APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2019
+      PYVER: 3
+      BITS: 64
 
 init:
   - "ECHO %PYTHON% %vcvars% (%bits%)"