code: def change_state(self, item, state):
          
          tags = self.item(item, "tags")
          states = ("checked", "unchecked", "tristate")
          new_tags = [t for t in tags if t not in states]
          new_tags.append(state)
          self.item(item, tags=tuple(new_tags))
docs: Replace the current state of the item.
  
          i.e. replace the current state tag but keeps the other tags.
          
          :param item: item id
          :type item: str
          :param state: "checked", "unchecked" or "tristate": new state of the item 
          :type state: str