use six.add_metaclass

Using six.add_metaclass instead of "__metaclass__" for Python 3.x
compatibility.

Change-Id: I8bf33fb1770511657587c8f7a33acfcf9a351e79

requirements.txt

@@ -1 +1,2 @@
 argparse
+six>=1.6.0

stevedore/example/base.py

@@ -1,12 +1,13 @@
 import abc
 
+import six
 
+
+@six.add_metaclass(abc.ABCMeta)
 class FormatterBase(object):
     """Base class for example plugin used in the tutoral.
     """
 
-    __metaclass__ = abc.ABCMeta
-
     def __init__(self, max_width=60):
         self.max_width = max_width