MILP: Formulating a sorted list for a constraint

Question

For a MILP project planning problem I would like to formulate the constraint that activity i must be finished before activity j starts. The activities are to be ordered by duration p and the one of three modes m per activity is to be used which takes the shortest time.

So I created a dictionary with the minimum durations of activity i in mode m.

p_im_min = {i: np.min([p[i,m] for m in M_i[i]]) for i in V}

Then I sorted the durations by size:

p_sort = (sorted(p_im_min.items(), key = lambda kv: kv[1]))

Which gives (i,p) in the right order:

p_sort = [(0, 0),
 (3, 1),
 (4, 1),
 (5, 1),
 (7, 1),
 (13, 1),
 (14, 1),
 (15, 1),
 (19, 1),
 (1, 2),
 (2, 2),
 (8, 2),
 (16, 2),
 (17, 2),
 (18, 2),
 (20, 2),
 (6, 3),
 (10, 3),
 (9, 4),
 (12, 4),
 (11, 5)] 

But now I want a list with (i,j), where i must always be finished before j starts. Since I could not find the function, I created this list manually, thus

order_act = [(0,3), (3,4), (4,5), (5,7), etc.

And finally (after formulating the parameters, variables and sets) added the following constraint:

mdl.addConstrs(y[i,j] == 1 
               for (i,j) in order_act)

My question:

Is there any way to use a formula/command in Python to create the list (i,j)? Because as it is now, it is not ideal and the solution is not satisfactory.