Add trove classifier for Python 3.6

Because the classifier was missing, the library cannot be installed on Python 3.6 because it can't be found.

Author: PCManticore

setup.py

@@ -43,6 +43,7 @@
         'Programming Language :: Python :: 3.3',
         'Programming Language :: Python :: 3.4',
         'Programming Language :: Python :: 3.5',
+        'Programming Language :: Python :: 3.6',
         'Programming Language :: Python',
         'Topic :: Internet :: Proxy Servers',
         'Topic :: Internet',