move exception class into shadow function as we may not want to export it...

move exception class into shadow function as we may not want to export it under the cython.* namespace at this point

Cython/Shadow.py

@@ -28,10 +28,6 @@ class _ArrayType(object):
         return "%s[%s]" % (self.dtype, ", ".join(axes))
 
 
-class InvalidTypeSpecification(Exception):
-    pass
-
-
 def index_type(base_type, item):
     """
     Support array type creation by slicing, e.g. double[:, :] specifies
@@ -40,6 +36,9 @@ def index_type(base_type, item):
     """
     assert isinstance(item, (tuple, slice))
 
+    class InvalidTypeSpecification(Exception):
+        pass
+
     def verify_slice(s):
         if s.start or s.stop or s.step not in (None, 1):
             raise InvalidTypeSpecification(