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__.

Added new requirement six >= 1.6.0.

Change-Id: Ic7e4f5cb03889cc6bfdb04a657d4654b5fedc985

designateclient/cli/base.py

@@ -20,10 +20,11 @@ from cliff.show import ShowOne
 from designateclient import exceptions
 from designateclient import utils
 from designateclient.v1 import Client
+import six
 
 
+@six.add_metaclass(abc.ABCMeta)
 class Command(CliffCommand):
-    __metaclass__ = abc.ABCMeta
 
     def run(self, parsed_args):
         client_args = {

designateclient/v1/base.py

@@ -14,17 +14,18 @@
 # License for the specific language governing permissions and limitations
 # under the License.
 import abc
+import six
 
 
+@six.add_metaclass(abc.ABCMeta)
 class Controller(object):
-    __metaclass__ = abc.ABCMeta
 
     def __init__(self, client):
         self.client = client
 
 
+@six.add_metaclass(abc.ABCMeta)
 class CrudController(Controller):
-    __metaclass__ = abc.ABCMeta
 
     @abc.abstractmethod
     def list(self, *args, **kw):

requirements.txt

@@ -3,4 +3,5 @@ jsonschema>=2.0.0,<3.0.0
 pbr>=0.6,<1.0
 python-keystoneclient>=0.6.0
 requests>=1.1
+six>=1.6.0
 stevedore>=0.14