title: "KeyError: 'PYTHONPATH'" message when updating Anaconda packahes on Mac OS X
body: <p>Each time I try to update my Anaconda (version 3.15.1) packages on my Mac (OS X 10.10.4), I get the following message: </p>
  
  <p><code>Error in sitecustomize; set PYTHONVERBOSE for traceback: 
  KeyError: 'PYTHONPATH'</code></p>
  
  <p>The rest of the updating process seems to run normally, but I'd like to figure out what causes this message and how to remove it.</p>