setup.py: declare support for py38

setup.py

@@ -869,6 +869,7 @@ 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',
     ],