A staticmethod that is used to determine the next value returned by auto:
_generate_next_value_(name, start, count, last_values)
>>> from enum import auto
>>> class PowersOfThree(Enum):
... @staticmethod
... def _generate_next_value_(name, start, count, last_values):
... return 3 ** (count + 1)
... FIRST = auto()
... SECOND = auto()
>>> PowersOfThree.SECOND.value
9