diff --git a/acme/acme/challenges.py b/acme/acme/challenges.py
index 96997297b..9702a7a14 100644
--- a/acme/acme/challenges.py
+++ b/acme/acme/challenges.py
@@ -9,6 +9,7 @@ from cryptography.hazmat.primitives import hashes  # type: ignore
 import josepy as jose
 import OpenSSL
 import requests
+import six
 
 from acme import errors
 from acme import crypto_util
@@ -139,6 +140,7 @@ class KeyAuthorizationChallengeResponse(ChallengeResponse):
         return True
 
 
+@six.add_metaclass(abc.ABCMeta)
 class KeyAuthorizationChallenge(_TokenChallenge):
     # pylint: disable=abstract-class-little-used,too-many-ancestors
     """Challenge based on Key Authorization.
@@ -147,7 +149,6 @@ class KeyAuthorizationChallenge(_TokenChallenge):
         that will be used to generate `response`.
 
     """
-    __metaclass__ = abc.ABCMeta
 
     response_cls = NotImplemented
     thumbprint_hash_function = (
diff --git a/certbot/interfaces.py b/certbot/interfaces.py
index f5858a7fd..c96f6bd51 100644
--- a/certbot/interfaces.py
+++ b/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.