pypi metadata: we also support python 3.8 now

tested regularly on travis-ci.

setup.py

@@ -269,6 +269,7 @@ setup(
         'Programming Language :: Python :: 3.5',
         'Programming Language :: Python :: 3.6',
         'Programming Language :: Python :: 3.7',
+        'Programming Language :: Python :: 3.8',
         'Topic :: Security :: Cryptography',
         'Topic :: System :: Archiving :: Backup',
     ],