IAttributeIndex broken under Python 3

[jamadden]

This is basically the same error as #5: field_name should be defined to be NativeStringLine instead of BytesLine.

This shows up testing zc.catalog:

      File "///python3.5/site-packages/zope/app/form/browser/add.py", line 111, in createAndAdd
        content = self.add(content)
      File "///python3.5/site-packages/zope/app/form/browser/add.py", line 134, in add
        return self.context.add(content)
      File "///python3.5/site-packages/zope/app/container/browser/adding.py", line 78, in add
        container[name] = content
      File "///python3.5/site-packages/zope/container/btree.py", line 102, in __setitem__
        setitem(self, self._setitemf, key, value)
      File "///python3.5/site-packages/zope/container/contained.py", line 571, in setitem
        notify(event)
      File "///python3.5/site-packages/zope/event/__init__.py", line 31, in notify
        subscriber(event)
      File "///python3.5/site-packages/zope/component/event.py", line 27, in dispatch
        component_subscribers(event, None)
      File "///python3.5/site-packages/zope/component/_api.py", line 139, in subscribers
        return sitemanager.subscribers(objects, interface)
      File "///python3.5/site-packages/zope/interface/registry.py", line 442, in subscribers
        return self.adapters.subscribers(objects, provided)
      File "///python3.5/site-packages/zope/interface/adapter.py", line 598, in subscribers
        subscription(*objects)
      File "///python3.5/site-packages/zope/component/event.py", line 36, in objectEventNotify
        component_subscribers((event.object, event), None)
      File "///python3.5/site-packages/zope/component/_api.py", line 139, in subscribers
        return sitemanager.subscribers(objects, interface)
      File "///python3.5/site-packages/zope/interface/registry.py", line 442, in subscribers
        return self.adapters.subscribers(objects, provided)
      File "///python3.5/site-packages/zope/interface/adapter.py", line 598, in subscribers
        subscription(*objects)
      File "///python3.5/site-packages/zope/catalog/catalog.py", line 173, in indexAdded
        index.__parent__.updateIndex(index)
      File "///python3.5/site-packages/zope/catalog/catalog.py", line 100, in updateIndex
        index.index_doc(uid, obj)
      File "///python3.5/site-packages/zope/catalog/attribute.py", line 130, in index_doc
        value = getattr(object, self.field_name, None)
    TypeError: getattr(): attribute name must be string