Remove usage of six

Remove six-library Replace the following items with Python 3 style code.
- six.add_metaclass

Change-Id: I7da3e8898e00bc07f6e462cbc9765ce7c199faa2
diff --git a/tempest/test_discover/plugins.py b/tempest/test_discover/plugins.py
index b20b60e..1d69d9d 100644
--- a/tempest/test_discover/plugins.py
+++ b/tempest/test_discover/plugins.py
@@ -15,7 +15,6 @@
 import abc
 
 from oslo_log import log as logging
-import six
 import stevedore
 
 from tempest.lib.common.utils import misc
@@ -24,8 +23,7 @@
 LOG = logging.getLogger(__name__)
 
 
-@six.add_metaclass(abc.ABCMeta)
-class TempestPlugin(object):
+class TempestPlugin(object, metaclass=abc.ABCMeta):
     """Provide basic hooks for an external plugin
 
     To provide tempest the necessary information to run the plugin.