Fix generation of indices in linear_assignment.py

[Thunfischpirat]

the sklearn function linear_assignment is deprecated and has to be replaced with linear_sum_assignment in linear_assignment.py like this:

from scipy.optimize import linear_sum_assignment as linear_assignment

Furthermore line 58 of the same file needs to be replaced by
indices = np.asarray(linear_assignment(cost_matrix)).T

Source

One of the proposed mergers #286 also fixed this problem. Please accept it.