add python 3.7 to pypi metadata

setup.py

@@ -277,6 +277,7 @@ setup(
         'Programming Language :: Python',
         'Programming Language :: Python :: 3',
         'Programming Language :: Python :: 3.6',
+        'Programming Language :: Python :: 3.7',
         'Topic :: Security :: Cryptography',
         'Topic :: System :: Archiving :: Backup',
     ],