Try fix appveyor build

Author: bakwc

appveyor.yml

@@ -5,7 +5,7 @@ environment:
     - PYTHON: "C:\\Python27"
 #    - PYTHON: "C:\\Python34"
     - PYTHON: "C:\\Python35"
-    - PYTHON: "C:\\Python39"
+    - PYTHON: "C:\\Python38"
 
 install:
   - "%PYTHON%\\python.exe -m pip install pytest"