Drop six usage

With python3, six is not needed anymore, drop it.

Change-Id: I70bb679270605ac32ca0cceb9414ea3a210e5842

diskimage_builder/block_device/plugin.py

@@ -12,7 +12,6 @@
 
 import abc
 import logging
-import six
 
 #
 # Plugins convert configuration entries into graph nodes ready for
@@ -22,8 +21,7 @@ import six
 logger = logging.getLogger(__name__)
 
 
-@six.add_metaclass(abc.ABCMeta)
-class NodeBase(object):
+class NodeBase(object, metaclass=abc.ABCMeta):
     """A configuration node entry
 
     This is the main driver class for dib-block-device operation.
@@ -155,8 +153,7 @@ class NodeBase(object):
         return
 
 
-@six.add_metaclass(abc.ABCMeta)
-class PluginBase(object):
+class PluginBase(object, metaclass=abc.ABCMeta):
     """The base plugin object
 
     This is the base plugin object.  Plugins are an instantiation of

lower-constraints.txt

@@ -36,7 +36,6 @@ PyYAML==3.12
 reno==2.5.0
 requests==2.14.2
 requestsexceptions==1.2.0
-six==1.10.0
 snowballstemmer==1.2.1
 Sphinx==1.6.2
 sphinxcontrib-websupport==1.0.1

requirements.txt

@@ -5,7 +5,6 @@
 networkx>=1.10 # BSD
 pbr!=2.1.0,>=2.0.0 # Apache-2.0
 PyYAML>=3.12 # MIT
-six>=1.10.0 # MIT
 stevedore>=1.20.0 # Apache-2.0
 # NOTE(ianw) in here because dib-lint uses flake8
 flake8<4.0.0,>=3.6.0 # MIT