List Python 3 support in setup.py.
diff --git a/setup.py b/setup.py
index 8a2df8d..e5a8570 100755
--- a/setup.py
+++ b/setup.py
@@ -22,6 +22,7 @@
         'License :: OSI Approved :: Apache Software License',
         'Operating System :: OS Independent',
         'Programming Language :: Python',
+        'Programming Language :: Python :: 3',
         'Topic :: Security :: Cryptography',
     ],
     install_requires=[