CI: Add Python 3.7 to AppVeyor matrix

Author: effigies

appveyor.yml

@@ -12,6 +12,8 @@ environment:
     - PYTHON: C:\Python35-x64
     - PYTHON: C:\Python36
     - PYTHON: C:\Python36-x64
+    - PYTHON: C:\Python37
+    - PYTHON: C:\Python37-x64
 
 install:
   # Prepend newly installed Python to the PATH of this build (this cannot be