at line 171 in csp.py, in the function AC3, it is written as :
if Xk != Xi:

there is no need to exclude Xi because Xi already can't be a neighbour to itself.
but I guess it should be
if Xk != Xj:
because Xi is already revised with Xj, so there is no need to add Xi, Xj to the queue again.

we can also see this in the book at Figure6.3 as :
for each Xk in Xi.NEIGHBORS - {Xj} do
here again, Xj is excluded.