title: cbook missing in fresh install of matplotlib
body: <p>I installed ipython and matplot lib via pip</p>
  
  <p>when I try to <code>import matplotlib.pyplot as plt</code>, I get this error:</p>
  
  <pre><code>bash-4.2$ ipython
  Python 3.6.3 (default, Jan  4 2018, 16:40:53) 
  Type 'copyright', 'credits' or 'license' for more information
  IPython 6.4.0 -- An enhanced Interactive Python. Type '?' for help.
  
  In [1]: import matplotlib.pyplot as plt
  ---------------------------------------------------------------------------
  ImportError                               
  Traceback (most recent call last)
  &lt;ipython-input-4-a0d2faabd9e9&gt; in &lt;module&gt;()
  ----&gt; 1 import matplotlib.pyplot as plt
  
  ~/.local/lib/python3.6/site-packages/matplotlib/__init__.py in &lt;module&gt;()
      125 # cbook must import matplotlib only within function
      126 # definitions, so it is safe to import from it here.
  --&gt; 127 from . import cbook
      128 from matplotlib.cbook import (
      129     _backports, mplDeprecation, dedent, get_label, sanitize_sequence)
  
  ImportError: cannot import name 'cbook'
  </code></pre>
  
  <p>How can I fix this?</p>