After defining my own __eq__ method for a class that judged equality
based on a 'name' variable, imagine my surprise to see this:

In [20]: my_graph.edges[-1].end == my_graph.vertices[-1]
Out [20]: True

In [21]: my_graph.edges[-1].end != my_graph.vertices[-1]
Out [21]: True

...all because I forgot to modify __ne__ as well.

I think __ne__ should be changed to 'not __eq__' for the sake of
consistency.