Make np.savetxt(filename, zip(...)) work in Python 3.x

In Python 3, the behavior of ``zip`` changed - it now returns an iterator instead of a list. Therefore, things like

```
np.savetxt(filename, zip(...))
```

and 

```
np.array(zip([1,2,3],[4,5,6]), dtype=[('a', int), ('b', int)])
```

don't work anymore unless one wraps ``list()`` around ``zip()``. This minor PR ensures that ``savetxt`` does work with the new ``zip()`` function, though I'm not sure how to fix the second case since that seems to be handled at the C-level, but I think that if the second could somehow be fixed, this would fix the first, because ``savetxt`` calls ``asarray``, which calls ``array``.