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Converts a many-to-one mapping into a one-to-many mapping.

many_to_one must be a dictionary whose keys and values are all hashable.

The return value is a dictionary mapping values from many_to_one to sets of keys from many_to_one that have that value.

For example:

>>> from networkx.utils import groups
>>> many_to_one = {'a': 1, 'b': 1, 'c': 2, 'd': 3, 'e': 3}
>>> groups(many_to_one)  
{1: {'a', 'b'}, 2: {'c'}, 3: {'d', 'e'}}