Apply six for metaclass

The way to using metaclass has changed in Python3.

Python 2.7 way:

class Foo(object):
  __metaclass__ = FooMeta

Python 3 way:

class Foo(object, metaclass=FooMeta):
  ...

The six.add_metaclass() decorator allows us to use one syntax that
works for both Python 2.7 and Python 3.

Change-Id: Ibd31a9f91bac6ec220d802f16c77ee0306e211b3
Closes-Bug: #1236648

novaclient/base.py

@@ -163,13 +163,12 @@ class Manager(utils.HookableMixin):
                 return self.resource_class(self, body)
 
 
+@six.add_metaclass(abc.ABCMeta)
 class ManagerWithFind(Manager):
     """
     Like a `Manager`, but with additional `find()`/`findall()` methods.
     """
 
-    __metaclass__ = abc.ABCMeta
-
     @abc.abstractmethod
     def list(self):
         pass