dok_matrix no longer allows update

[jkroenke]

In the file network.py in the _mpi_nsi_arenas_betweenness function the line 3712:

sp_Pi.update(zip(update_keys, update_vals))

raises a NotImplementedError because dok_matrix no longer allows update (See this: https://github.com/scipy/scipy/issues/8338)

[ntfrgl]

I would use fancy indexing assignment, as suggested in the issue. That should be a one-line change.