Make module Python3 compatible.

[lmcinnes]

The python module is compatible only with Python2. I have made the minor necessary changes to simultaneously support both Python3 and Python2.

[wlbksy]

it works

[LGro]

Thanks for the efforts.
Compiling with gcc (GCC) 4.8.5 and installing under Python 3.6.2 :: Continuum Analytics, Inc. I get the following error on import:

ImportError: dynamic module does not define module export function (PyInit_LargeVis)

The solution to this seems to be renaming the function PyInit_myextension(void) in LargeVismodule.cpp:171 to PyInit_LargeVis(void), alongside with the matching change in line 158, where it also still says myextension.
@lmcinnes can you validate this and update your pull request?