In Python 3.6 the following works:

class HexInt(int):
    def __repr__(self):
        return hex(self)

class MyEnum(HexInt, enum.Enum):
    A = 1
    B = 2
    C = 3

>>> MyEnum.A
<MyEnum.A: 0x1>

However in Python 3.7/8 it instead prints
>>> MyEnum.A
0x1

It uses HexInt's repr instead of Enum's. Looking at the enum.py module it seems that this occurs for mixin classes that don't define __new__ due to a change in the _get_mixins_ method. If I define a __new__ method on the HexInt class then the expected behavior occurs.