Change __metaclass__ with optimal six.add_metaclass

Use six.add_metaclass instead of __metaclass__.[1]

[1]http://docs.openstack.org/developer/hacking/#

Change-Id: I4a6e5dc029bab6196256f4e95678e7d3939c3f62

tox.ini

@@ -32,11 +32,10 @@ show-source = True
 #E302: expected 2 blank linee
 #E303: too many blank lines (2)
 #H233: Python 3.x incompatible use of print operator
-#H236: Python 3.x incompatible __metaclass__, use six.add_metaclass()
 #H302: import only modules.
 #H404: multi line docstring should start without a leading new line
 #H405: multi line docstring summary not separated with an empty line
 #H904: Wrap long lines in parentheses instead of a backslash
-ignore = E123,E125,H803,E302,E303,H233,H236,H302,H404,H405,H904
+ignore = E123,E125,H803,E302,E303,H233,H302,H404,H405,H904
 builtins = _
 exclude=.venv,.git,.tox,dist,doc,*openstack/common*,*lib/python*,*egg,build

vmtp/perf_tool.py

@@ -15,6 +15,7 @@
 
 import abc
 import re
+import six
 
 from log import LOG
 from pkg_resources import resource_filename
@@ -25,8 +26,8 @@ SCP_DEST_DIR = '/tmp/'
 #
 # A base class for all tools that can be associated to an instance
 #
+@six.add_metaclass(abc.ABCMeta)
 class PerfTool(object):
-    __metaclass__ = abc.ABCMeta
 
     def __init__(self, name, instance):
         self.name = name