Added "Python 3 Only" trove classifier.

setup.py

@@ -64,6 +64,7 @@ setup(
         'Programming Language :: Python :: 3',
         'Programming Language :: Python :: 3.5',
         'Programming Language :: Python :: 3.6',
+        'Programming Language :: Python :: 3 :: Only',
         'Topic :: Internet :: WWW/HTTP',
         'Topic :: Internet :: WWW/HTTP :: Dynamic Content',
         'Topic :: Internet :: WWW/HTTP :: WSGI',