title: Import cElementTree failed with pyenv and python 2.7.12
body: <p>I have tried to import cElementTree, but failed.</p>
  
  <pre><code>Python 2.7.12 (default, Jul 19 2018, 15:12:44)
  [GCC 4.8.4] on linux2
  Type "help", "copyright", "credits" or "license" for more information.
  &gt;&gt;&gt; import xml.etree.cElementTree
  Traceback (most recent call last):
    File "&lt;stdin&gt;", line 1, in &lt;module&gt;
    File "/home/tomcat/.pyenv/versions/2.7.12/lib/python2.7/xml/etree/cElementTree.py", line 3, in &lt;module&gt;
      from _elementtree import *
  ImportError: No module named _elementtree
  </code></pre>
  
  <p>It looks <code>_elementtree</code> module (C lang module ?) doesn't exist in pyenv environment. What should I do ?</p>