appveyor: Add python 3.8 to environment matrix

Author: artembo

appveyor.yml

@@ -16,6 +16,8 @@ environment:
     - PYTHON: "C:\\Python36-x64"
     - PYTHON: "C:\\Python37"
     - PYTHON: "C:\\Python37-x64"
+    - PYTHON: "C:\\Python38"
+    - PYTHON: "C:\\Python38-x64"
 
 install:
   # install runtime dependencies