title: Creating a sandbox in virtualenv with python 2.7 fails on ubuntu?
body: <p>Why is virtualenv complaining about missing <code>_weakrefset</code>  </p>
  
  <pre><code>laptop:~/sandbox$ virtualenv --python=/usr/bin/python2.7 --no-site-packages play
  Running virtualenv with interpreter /usr/bin/python2.7
  New python executable in play/bin/python2.7
  Also creating executable in play/bin/python
  Traceback (most recent call last):
    File "/home/ribery/sandbox/play/lib/python2.7/site.py", 
      line 67, in &lt;module&gt;
    import os
    File "/home/ribery/sandbox/play/lib/python2.7/os.py", line 398, 
      in &lt;module&gt;
    import UserDict
  File "/home/ribery/sandbox/play/lib/python2.7/UserDict.py", line 83, 
      in &lt;module&gt;
    import _abcoll
  File "/home/ribery/sandbox/play/lib/python2.7/_abcoll.py", line 11, 
      in &lt;module&gt;
    from abc import ABCMeta, abstractmethod
  File "/home/ribery/sandbox/play/lib/python2.7/abc.py", line 8, 
      in &lt;module&gt;
    from _weakrefset import WeakSet
  ImportError: No module named _weakrefset
  ERROR: The executable django-py27/bin/python2.7 is not functioning
  ERROR: It thinks sys.prefix is '/home/ribery/sandbox' 
  (should be '/home/ribery/sandbox/play')
  ERROR: virtualenv is not compatible with this system or executable
  </code></pre>