classes/lib: Complete transition to python3

This patch contains all the other misc pieces of the transition to
python3 which didn't make sense to be broken into individual patches.

(From OE-Core rev: fcd6b38bab8517d83e1ed48eef1bca9a9a190f57)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/lib/oe/classutils.py

@@ -1,4 +1,11 @@
-class ClassRegistry(type):
+
+class ClassRegistryMeta(type):
+    """Give each ClassRegistry their own registry"""
+    def __init__(cls, name, bases, attrs):
+        cls.registry = {}
+        type.__init__(cls, name, bases, attrs)
+
+class ClassRegistry(type, metaclass=ClassRegistryMeta):
     """Maintain a registry of classes, indexed by name.
 
 Note that this implementation requires that the names be unique, as it uses
@@ -12,12 +19,6 @@ Subclasses of ClassRegistry may define an 'implemented' property to exert
 control over whether the class will be added to the registry (e.g. to keep
 abstract base classes out of the registry)."""
     priority = 0
-    class __metaclass__(type):
-        """Give each ClassRegistry their own registry"""
-        def __init__(cls, name, bases, attrs):
-            cls.registry = {}
-            type.__init__(cls, name, bases, attrs)
-
     def __init__(cls, name, bases, attrs):
         super(ClassRegistry, cls).__init__(name, bases, attrs)
         try: