Fixup py3 ABCMeta abstract classes

kafka/consumer/subscription_state.py

@@ -497,7 +497,7 @@ def is_fetchable(self):
         return not self.paused and self.has_valid_position
 
 
-class ConsumerRebalanceListener(object, metaclass=abc.ABCMeta):
+class ConsumerRebalanceListener(metaclass=abc.ABCMeta):
     """
     A callback interface that the user can implement to trigger custom actions
     when the set of partitions assigned to the consumer changes.

kafka/coordinator/assignors/abstract.py

@@ -5,7 +5,7 @@
 )
 
 
-class AbstractPartitionAssignor:
+class AbstractPartitionAssignor(metaclass=abc.ABCMeta):
     """
     Abstract assignor implementation which does some common grunt work (in particular collecting
     partition counts which are always needed in assignors).

kafka/coordinator/base.py

@@ -57,7 +57,7 @@ class UnjoinedGroupException(Errors.KafkaError):
     retriable = True
 
 
-class BaseCoordinator:
+class BaseCoordinator(metaclass=abc.ABCMeta):
     """
     BaseCoordinator implements group management for a single group member
     by interacting with a designated Kafka broker (the coordinator). Group

kafka/metrics/measurable.py

@@ -1,7 +1,7 @@
 import abc
 
 
-class AbstractMeasurable:
+class AbstractMeasurable(metaclass=abc.ABCMeta):
     """A measurable quantity that can be registered as a metric"""
     @abc.abstractmethod
     def measure(self, config, now):