use inspect.signature for decorators

fixes arskom#538

spyne/decorator.py

@@ -29,7 +29,7 @@
 import spyne.const.xml
 
 from copy import copy
-from inspect import isclass
+from inspect import isclass, signature
 
 from spyne import MethodDescriptor
 
@@ -58,10 +58,9 @@ def _produce_input_message(f, params, in_message_name, in_variable_names,
 
     if argnames is None:
         try:
-            argcount = f.__code__.co_argcount
-            argnames = f.__code__.co_varnames[arg_start:argcount]
+            argnames = tuple(signature(f).parameters)[arg_start:]
 
-        except AttributeError:
+        except ValueError:
             raise TypeError(
                 "It's not possible to instrospect builtins. You must pass a "
                 "sequence of argument names as the '_args' argument to the "