AttributeError: 'module' object has no attribute 'next'

Came across this error while testing on my Ubuntu 12.02 LTS machine with a system install of six (version 1.1.0)

```
Traceback (most recent call last):
  File "2dcollections3d_demo.py", line 10, in <module>
    ax.plot(x, y, zs=0, zdir='z', label='zs=0, zdir=z')
  File "/home/ben/Programs/matplotlib/lib/mpl_toolkits/mplot3d/axes3d.py", line 1518, in plot
    lines = Axes.plot(self, xs, ys, *args[argsi:], **kwargs)
  File "/home/ben/Programs/matplotlib/lib/matplotlib/axes/_axes.py", line 1394, in plot
    for line in self._get_lines(*args, **kwargs):
  File "/home/ben/Programs/matplotlib/lib/matplotlib/axes/_base.py", line 303, in _grab_next_args
    for seg in self._plot_args(remaining, kwargs):
  File "/home/ben/Programs/matplotlib/lib/matplotlib/axes/_base.py", line 291, in _plot_args
    seg = func(x[:, j % ncx], y[:, j % ncy], kw, kwargs)
  File "/home/ben/Programs/matplotlib/lib/matplotlib/axes/_base.py", line 236, in _makeline
    kw['color'] = six.next(self.color_cycle)
AttributeError: 'module' object has no attribute 'next'
```
Perhaps we need to set a minimum version of six?