Use six.add_metaclass instead of __metaclass__

According to http://docs.openstack.org/developer/hacking/ it should
be preferred to use six.add_metaclass instead of __metaclass__.

Change-Id: I33a0d1b0a0a31beacc8c768eb18fac3cb9ed5936

cliff/command.py

@@ -3,15 +3,16 @@ import abc
 import argparse
 import inspect
 
+import six
 
+
+@six.add_metaclass(abc.ABCMeta)
 class Command(object):
     """Base class for command plugins.
 
     :param app: Application instance invoking the command.
     :paramtype app: cliff.app.App
     """
-    __metaclass__ = abc.ABCMeta
-
     def __init__(self, app, app_args):
         self.app = app
         self.app_args = app_args

cliff/display.py

@@ -3,6 +3,7 @@
 import abc
 import logging
 
+import six
 import stevedore
 
 from .command import Command
@@ -11,10 +12,10 @@ from .command import Command
 LOG = logging.getLogger(__name__)
 
 
+@six.add_metaclass(abc.ABCMeta)
 class DisplayCommandBase(Command):
     """Command base class for displaying data about a single object.
     """
-    __metaclass__ = abc.ABCMeta
 
     def __init__(self, app, app_args):
         super(DisplayCommandBase, self).__init__(app, app_args)

cliff/formatters/base.py

@@ -3,9 +3,11 @@
 
 import abc
 
+import six
 
+
+@six.add_metaclass(abc.ABCMeta)
 class Formatter(object):
-    __metaclass__ = abc.ABCMeta
 
     @abc.abstractmethod
     def add_argument_group(self, parser):
@@ -15,10 +17,10 @@ class Formatter(object):
         """
 
 
+@six.add_metaclass(abc.ABCMeta)
 class ListFormatter(Formatter):
     """Base class for formatters that know how to deal with multiple objects.
     """
-    __metaclass__ = abc.ABCMeta
 
     @abc.abstractmethod
     def emit_list(self, column_names, data, stdout, parsed_args):
@@ -32,10 +34,10 @@ class ListFormatter(Formatter):
         """
 
 
+@six.add_metaclass(abc.ABCMeta)
 class SingleFormatter(Formatter):
     """Base class for formatters that work with single objects.
     """
-    __metaclass__ = abc.ABCMeta
 
     @abc.abstractmethod
     def emit_one(self, column_names, data, stdout, parsed_args):

cliff/lister.py

@@ -13,16 +13,18 @@ except ImportError:
 
 import logging
 
+import six
+
 from .display import DisplayCommandBase
 
 
 LOG = logging.getLogger(__name__)
 
 
+@six.add_metaclass(abc.ABCMeta)
 class Lister(DisplayCommandBase):
     """Command base class for providing a list of data as output.
     """
-    __metaclass__ = abc.ABCMeta
 
     @property
     def formatter_namespace(self):

cliff/show.py

@@ -4,16 +4,18 @@ import abc
 import itertools
 import logging
 
+import six
+
 from .display import DisplayCommandBase
 
 
 LOG = logging.getLogger(__name__)
 
 
+@six.add_metaclass(abc.ABCMeta)
 class ShowOne(DisplayCommandBase):
     """Command base class for displaying data about a single object.
     """
-    __metaclass__ = abc.ABCMeta
 
     @property
     def formatter_namespace(self):