wic: pluginbase: use python 3 metaclass syntax

Used more readable syntax to specify metaclass for
the base plugin classes.

(From OE-Core rev: 808451f154867caef73e00af04f56f051b7d5f6b)

Signed-off-by: Ed Bartosh <ed.bartosh@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

scripts/lib/wic/pluginbase.py

@@ -32,10 +32,10 @@ class PluginMeta(type):
 
         return class_type
 
-class ImagerPlugin(PluginMeta("Plugin", (), {})):
+class ImagerPlugin(metaclass=PluginMeta):
     wic_plugin_type = "imager"
 
-class SourcePlugin(PluginMeta("Plugin", (), {})):
+class SourcePlugin(metaclass=PluginMeta):
     wic_plugin_type = "source"
     """
     The methods that can be implemented by --source plugins.