Update classifiers, use better assertion messages in tests

Author: multimeric

setup.py

@@ -13,11 +13,10 @@
     license='MIT',
     classifiers=[
         'Framework :: Flask',
-        'Programming Language :: Python :: 2',
-        'Programming Language :: Python :: 2.7',
         'Programming Language :: Python :: 3',
         'Programming Language :: Python :: 3.4',
         'Programming Language :: Python :: 3.5',
+        'Programming Language :: Python :: 3.6',
         'License :: OSI Approved :: MIT License',
     ],
     keywords='web api rest jsonapi flask sqlalchemy marshmallow',