Add tuple key support in `GenericGraph.__getitem__()`

[HairlessVillager]

Description of proposed feature

vertices = [1, 2, 3, 4]
edges = [(1, 2), (2, 3), (3, 4), (1, 3), (1, 4)]
g = Graph(vertices, edges)
assert isinstance(g[1], Dot)  # This is OK
assert isinstance(g[(1, 2)], Line)  # But this will raise a KeyError

imo This is confusing. Why not add a support for edge mapping?

How can the new feature be used?

Every time people want to get or set a edge, just write g[(1, 2)] instead of g.edges[(1, 2)].