bpo-45624: make test_graphlib not depend on the iteration order of sets (GH-29233) (GH-29293)

the current test depended on integer sets being iterated on in a certain
fixed order. That order is different on PyPy (insertion based) and could
change in CPython in the future in theory. Make the test robust against
a different iteration order by sorting.
(cherry picked from commit 7401694807fc6b5f7b35ff73c06f4bb852e02946)

Co-authored-by: Carl Friedrich Bolz-Tereick <cfbolz@gmx.de>