Update appveyor.yml for Python 3.8

Author: TheGreatCabbage

appveyor.yml

@@ -1,8 +1,8 @@
 environment:
   matrix:
-    - PYTHON: "C:\\Python35"
     - PYTHON: "C:\\Python36"
     - PYTHON: "C:\\Python37"
+    - PYTHON: "C:\\Python38"
 
 init:
   - set PATH=%PYTHON%;%PYTHON%\Scripts;%PATH%