If a PolymorphicSerializer is subclassed

PolymorphicSerializerMetaclass.__new__ raises an error because discriminator does not exist in attrs

    def __new__(cls, name, bases, attrs):
        attrs["discriminator"] = cls._sanitize_discriminator(name, attrs)
        return super().__new__(cls, name, bases, attrs)

currently fixed this by copying the discriminator:

class RolSubSerializer(SubSerializerMixin, RolSerializer):
    discriminator = RolSerializer.discriminator

Another issue is when the discriminatior has set the group_field, this field is added to all fields in the mapping in PolymorphicSerializerMetaclass.santize_discriminator, when the subclass (that copied the discriminator) is loaded this function is called again which results in <group_field><group_field>, this can currently be fixed by copying the mapping to the original serializer and the subserializer or by not setting group_field on the sub_serializer.