Cython Enums can't be pickled

[fxjung]

see e.g.

cls = <class 'multiprocessing.reduction.ForkingPickler'>, obj = <concurrent.futures.process._CallItem object at 0x7f266ab446a0>, protocol = None

    @classmethod
    def dumps(cls, obj, protocol=None):
        buf = io.BytesIO()
>       cls(buf, protocol).dump(obj)
E       _pickle.PicklingError: Can't pickle <enum 'ExternalCost'>: attribute lookup ExternalCost on importlib._bootstrap failed

/home/jung/anaconda3/envs/the_simulator_b/lib/python3.9/multiprocessing/reduction.py:51: PicklingError