add python 3.7 to pypi metadata

(cherry picked from commit 751cd2caedf8d59857c0a4ebc76e54b04196e342)

setup.py

@@ -830,6 +830,7 @@ setup(
         'Programming Language :: Python :: 3.4',
         'Programming Language :: Python :: 3.5',
         'Programming Language :: Python :: 3.6',
+        'Programming Language :: Python :: 3.7',
         'Topic :: Security :: Cryptography',
         'Topic :: System :: Archiving :: Backup',
     ],