code: def config_items(self, sortkey = False):
          
          if sortkey:
              items = sorted(self.items())
          else:
              items = self.items()
          for k,v in items:
              if isinstance(v, ConfigTree):
                  for k2,v2 in v.config_items(sortkey):
                      yield (k + '.' + k2, v2)
              else:
                  yield (k,v)
docs: Return all `(key, value)` tuples for configurations in this node, including configurations on children nodes.