[3.11] gh-103479: [Enum] require __new__ to be considered a data type (GH-103495) (GH-103514)

a mixin must either have a __new__ method, or be a dataclass, to be interpreted as a data-type; an __init__ method is not enough (restores pre-3.11 behavior for non-dataclasses).

(cherry picked from commit a6f95941a3d686707fb38e0f37758e666f25e180)
Co-authored-by: Ethan Furman <ethan@stoneleaf.us>