title: import Axes3D issues
body: <p>I'm using python 3.5 and i tried to import Axes3D from mpl_toolkitd.mplot3d but unfortunately i get this error:</p>
  
  <pre><code>In [1]: from mpl_toolkits.mplot3d import Axes3D
  ---------------------------------------------------------------------------
  ImportError                               Traceback (most recent call last)
  &lt;ipython-input-1-357f877dbd2b&gt; in &lt;module&gt;()
  ----&gt; 1 from mpl_toolkits.mplot3d import Axes3D
  
  /usr/lib/python3/dist-packages/mpl_toolkits/mplot3d/__init__.py in &lt;module&gt;()
        2                         unicode_literals)
        3 
  ----&gt; 4 from matplotlib.externals import six
        5 
        6 from .axes3d import Axes3D
  
  ImportError: No module named 'matplotlib.externals'
  </code></pre>