py23 Use new-style classes

Such that we get the same semantics in both Python 2 and 3.
diff --git a/Lib/fontTools/afmLib.py b/Lib/fontTools/afmLib.py
index 9ef5951..ac31563 100644
--- a/Lib/fontTools/afmLib.py
+++ b/Lib/fontTools/afmLib.py
@@ -83,7 +83,7 @@
 class error(Exception): pass
 
 
-class AFM:
+class AFM(object):
 	
 	_attrs = None