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With the landing of ordered class namespaces the build order for Enum needs to be redone to at least keep the user-specified pieces' relative order intact:
>>> from enum import Enum
>>> class Color(Enum):
... red = 1
... blue = 2
... green = 3
... def primary(self):
... return True
... 
>>> Color.__dict__.keys()
dict_keys(['_generate_next_value_', '__module__', 'primary', '__doc__', '_member_names_', '_member_map_', '_member_type_', '_value2member_map_', 'red', 'blue', 'green', '__new__'])

__definition_order__ was dropped from PEP520, so this issue is moot.