BUG: Fix 'TypeError: expected bytes, str found' on Python 3

[natgrid\test.py](https://github.com/matplotlib/natgrid/blob/master/test.py) fails on Python 3 because [`_natgrid.seti`](https://github.com/matplotlib/natgrid/blob/master/src/_natgrid.pyx#L32) and [`_natgrid.setr`](https://github.com/matplotlib/natgrid/blob/master/src/_natgrid.pyx#L38) expect byte strings:
```
Traceback (most recent call last):
  File "test.py", line 15, in <module>
    zi = griddata(x,y,z,xi,yi)
  File "X:\Python34\lib\site-packages\matplotlib\mlab.py", line 2586, in griddata
    _natgrid.seti('ext',0)
  File "_natgrid.pyx", line 35, in _natgrid.seti (src\_natgrid.c:875)
TypeError: expected bytes, str found
```