If nest is not a structure , tuple (or a namedtuple), dict, or an attrs class,
then returns a single-element list:
[nest].
In the case of dict instances, the sequence consists of the values, sorted by
key to ensure deterministic behavior. This is true also for OrderedDict
instances: their sequence order is ignored, the sorting order of keys is used
instead. The same convention is followed in pack_sequence_as. This correctly
repacks dicts and OrderedDicts after they have been flattened, and also allows
flattening an OrderedDict and then repacking it back using a corresponding
plain dict, or vice-versa. Dictionaries with non-sortable keys cannot be
flattened.
Users must not modify any collections used in nest while this function is
running.