Add missing dependency on "six"

Author: dinvlad

setup.py

@@ -40,6 +40,7 @@
         'flask >= 1.1.2',
         'python-hosts >= 1.0.0',
         'requests >= 2.24.0',
+        'six >= 1.15.0',
         'uwsgi >= 2.0.19.1',
         'yubikey-manager >= 3.1.1',
     ],