correct metaclass usage everywhere (#5919)

certbot/interfaces.py

@@ -7,11 +7,10 @@ import zope.interface
 # pylint: disable=too-few-public-methods
 
 
+@six.add_metaclass(abc.ABCMeta)
 class AccountStorage(object):
     """Accounts storage interface."""
 
-    __metaclass__ = abc.ABCMeta
-
     @abc.abstractmethod
     def find_all(self):  # pragma: no cover
         """Find all accounts.