Turn on AppVeyor testing for Python 2.7, Python 3.6 and Python 3.7

Author: rpep

appveyor.yml

@@ -4,10 +4,10 @@ skip_branch_with_pr: true
 # environment variables
 environment:
   matrix:
+    - PYTHON: "C:\\Python27-x64"
     - PYTHON: "C:\\Python35-x64"
-      PYTHON_VERSION: "3.5.x"
-      PYTHON_MAJOR: 3
-      PYTHON_ARCH: "64"
+    - PYTHON: "C:\\Python36-x64"
+    - PYTHON: "C:\\Python37-x64"
 
 # scripts that run after cloning repository
 install: