Mock dictionary unpacking operator (**)

How can one mock the dictionary unpacking operator (**)?

Expected pseudo Python code:

from unittest import mock

a = mock.MagicMock()
a.__unpack__.return_value = {'foo': 'bar'} # this is pseudo code

b = {**a}
print(b)

>>> {'foo': 'bar'}

Can this be done? I've found this question that suggests overloading __getitem__ is the way to go, but that doesn't seem to work for mocking.