Python 3 compatibility

[emengd]

Hi,

The python3 package found on pypi doesn't work for me, because the imports aren't python3-compliant.

import numpy as np
import _maxflow
from _maxflow import GraphInt, GraphFloat, moore_structure, vonNeumann_structure
from version import __version__, __version_str__, __version_core__

It works with :

import numpy as np
from . import _maxflow
from ._maxflow import GraphInt, GraphFloat, moore_structure, vonNeumann_structure
from .version import __version__, __version_str__, __version_core__

Edit : I made a pull request.