docs: def load(cls, data):
          """Construct a Constant class from it's dict data.
  
          .. versionadded:: 0.0.2
          """
          if len(data) == 1:
              for key, value in data.items():
                  if "__classname__" not in value:  # pragma: no cover
                      raise ValueError
                  name = key
                  bases = (Constant,)
                  attrs = dict()
                  for k, v in value.items():
                      if isinstance(v, dict):
                          if "__classname__" in v:
                              attrs[k] = cls.load({k: v})
                          else:
                              attrs[k] = v
                      else:
                          attrs[k] = v
              return type(name, bases, attrs)
          else:  # pragma: no cover
              raise ValueError