Python 3 compatibility

[jllanfranchi]

The c extension kde.c apparently calls a non-existent method Py_InitModule... Running via PISA's kde_hist.py module:

$ python pisa/utils/kde_hist.py 
Traceback (most recent call last):
  File "pisa/utils/kde_hist.py", line 8, in <module>
    from kde.cudakde import gaussian_kde
  File "/home/justin/anaconda2/envs/py36/lib/python3.6/site-packages/kde/cudakde.py", line 31, in <module>
    from .classes import KDE
  File "/home/justin/anaconda2/envs/py36/lib/python3.6/site-packages/kde/classes.py", line 34, in <module>
    from .kde import getLambda_ND, kde_ND
ImportError: /home/justin/anaconda2/envs/py36/lib/python3.6/site-packages/kde/kde.cpython-36m-x86_64-linux-gnu.so: undefined symbol: Py_InitModule

Possibly helpful: http://python3porting.com/cextensions.html