Apply six for metaclass

__metaclass__ cannot be used in python3.six be used in general
for python 3 compatibility.

Porting Change-Id I9fc7a59df3af29b4cc1287c40fa4e883d994a961
from oslo-incubator

Change-Id: Icdacdcf5556b6d3b8450d1350c6f62b4f5a9690b

oslo/messaging/_drivers/base.py

@@ -15,6 +15,8 @@
 
 import abc
 
+import six
+
 from oslo.messaging import exceptions
 
 
@@ -22,10 +24,9 @@ class TransportDriverError(exceptions.MessagingException):
     """Base class for transport driver specific exceptions."""
 
 
+@six.add_metaclass(abc.ABCMeta)
 class IncomingMessage(object):
 
-    __metaclass__ = abc.ABCMeta
-
     def __init__(self, listener, ctxt, message):
         self.conf = listener.conf
         self.listener = listener
@@ -37,10 +38,9 @@ class IncomingMessage(object):
         "Send a reply or failure back to the client."
 
 
+@six.add_metaclass(abc.ABCMeta)
 class Listener(object):
 
-    __metaclass__ = abc.ABCMeta
-
     def __init__(self, driver, target):
         self.conf = driver.conf
         self.driver = driver
@@ -51,10 +51,9 @@ class Listener(object):
         "Blocking until a message is pending and return IncomingMessage."
 
 
+@six.add_metaclass(abc.ABCMeta)
 class BaseDriver(object):
 
-    __metaclass__ = abc.ABCMeta
-
     def __init__(self, conf, url,
                  default_exchange=None, allowed_remote_exmods=[]):
         self.conf = conf

oslo/messaging/_drivers/pool.py

@@ -17,7 +17,10 @@ import abc
 import collections
 import threading
 
+import six
 
+
+@six.add_metaclass(abc.ABCMeta)
 class Pool(object):
 
     """A thread-safe object pool.
@@ -28,8 +31,6 @@ class Pool(object):
     Resizing is not supported.
     """
 
-    __metaclass__ = abc.ABCMeta
-
     def __init__(self, max_size=4):
         super(Pool, self).__init__()
 

oslo/messaging/_executors/base.py

@@ -16,15 +16,16 @@ import abc
 import logging
 import sys
 
+import six
+
 from oslo import messaging
 
 _LOG = logging.getLogger(__name__)
 
 
+@six.add_metaclass(abc.ABCMeta)
 class ExecutorBase(object):
 
-    __metaclass__ = abc.ABCMeta
-
     def __init__(self, conf, listener, callback):
         self.conf = conf
         self.listener = listener

oslo/messaging/notify/notifier.py

@@ -19,6 +19,7 @@ import abc
 import logging
 
 from oslo.config import cfg
+import six
 from stevedore import named
 
 from oslo.messaging.openstack.common import timeutils
@@ -39,10 +40,9 @@ _notifier_opts = [
 _LOG = logging.getLogger(__name__)
 
 
+@six.add_metaclass(abc.ABCMeta)
 class _Driver(object):
 
-    __metaclass__ = abc.ABCMeta
-
     def __init__(self, conf, topics, transport):
         self.conf = conf
         self.topics = topics

oslo/messaging/serializer.py

@@ -18,10 +18,12 @@ __all__ = ['Serializer', 'NoOpSerializer']
 
 import abc
 
+import six
 
+
+@six.add_metaclass(abc.ABCMeta)
 class Serializer(object):
     """Generic (de-)serialization definition base class."""
-    __metaclass__ = abc.ABCMeta
 
     @abc.abstractmethod
     def serialize_entity(self, ctxt, entity):