py3: fix doctest in generic_graph (part 1)

[dcoudert]

We fix doctests in

• incidence_matrix: we change the example from CubeGraph to PetersenGraph as it is stable for py2 and py3. It is possible to fix the order of the rows of the matrix with parameter vertices, but the order of the columns depends on .edge_iterator() which itself depends on .vertex_iterator() which is not always the same in py3 than in py2. An alternative would be to add parameter edges to the method to specify the ordering of the columns...

• eulerian_circuit: we fix the issue by changing the way the graph. It changes the internal order of vertices and edges and this is sufficient for getting same result in py2 and py3. With original construction, the internal order of vertices was depending on the order of the keys of the input dict (might be different in py2 and py3)

• min_spanning_tree: we sort the edges of the solution.

[dcoudert]

New commits:

​9f3c0eb

trac #27176: doctests in generic_graph part 1