fix 3.9 not used in appveyor jobs

Signed-off-by: oleg.hoefling <oleg.hoefling@gmail.com>

Author: hoefling

.appveyor.yml

@@ -11,7 +11,9 @@ environment:
   - python: 38
   - python: 38-x64
   - python: 39
+    image: Visual Studio 2019
   - python: 39-x64
+    image: Visual Studio 2019
 
 install:
   - SET PATH=C:\\Python%PYTHON%;c:\\Python%PYTHON%\\scripts;%PATH%