setup.py: add Python 3.6 qualifier

setup.py

@@ -292,6 +292,7 @@ setup(
         'Programming Language :: Python :: 3',
         'Programming Language :: Python :: 3.4',
         'Programming Language :: Python :: 3.5',
+        'Programming Language :: Python :: 3.6',
         'Topic :: Security :: Cryptography',
         'Topic :: System :: Archiving :: Backup',
     ],