Fix sankey.py pep8 and py3 compatibility

[cgohlke]

No description provided.

[pelson]

I've gone through these changes and they are fine. I have made similar stylistic improvements in the past, but we do need to be careful in making large scale changes to files whilst we have the v1.1.x branch in a non-frozen state (see #910).

This gets my +1.

[cgohlke]

That's a good point (making large scale changes). This could wait until after 1.1.1.

[pelson]

This could wait until after 1.1.1.

Lets do that. That is a whole other issue which I intend to bring up in the next couple of days :-)

[mdboom]

I agree about holding off on this until the 1.1.1 release.

It might also be nice, if possible, to separate out the actual fixes for Python 3 from the general PEP8 stuff.

[WeatherGod]

Now that v1.1.1 is out, would it be safe to get this merged in?

[pelson]

@WeatherGod: If your happy with the changes too, then I would say nothing.

[WeatherGod]

ok, merging....