title: Upgrade Pip error message
body: <p>When i run </p>
  
  <pre><code>    pip install --upgrade pip
  </code></pre>
  
  <p>I get this error message:</p>
  
  <pre><code>    Collecting pip
      Downloading pip-8.1.0-py2.py3-none-any.whl (1.2MB)
      100% |████████████████████████████████| 1.2MB 371kB/s 
      Installing collected packages: pip
      Found existing installation: pip 8.0.2
      Uninstalling pip-8.0.2:
      Exception:
      Traceback (most recent call last):
      File "/Library/Python/2.7/site-packages/pip/basecommand.py", line       209, in main
      status = self.run(options, args)
      File "/Library/Python/2.7/site-packages/pip/commands/install.py", line 317, in run
      prefix=options.prefix_path,
      File "/Library/Python/2.7/site-packages/pip/req/req_set.py", line  725, in install
      requirement.uninstall(auto_confirm=True)
      File "/Library/Python/2.7/site-packages/pip/req/req_install.py", line 752, in uninstall
      paths_to_remove.remove(auto_confirm)
      File "/Library/Python/2.7/site-packages/pip/req/req_uninstall.py",line 115, in remove
      renames(path, new_path)
      File "/Library/Python/2.7/site-packages/pip/utils/__init__.py", line 266,   in renames
      shutil.move(old, new)
      File"/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/shutil.py", line 303, in move
      os.unlink(src)
      OSError: [Errno 13] Permission denied: '/Library/Python/2.7/site- packages/pip-8.0.2.dist-info/DESCRIPTION.rst'
  </code></pre>
  
  <p>Previously I had been struggling to install and run a couple of python modules so I remember moving files around a bit. Is that what has caused this error? How can I fix this? I am on Mac. </p>
  
  <p>I was trying to install bs4 prior to this and I got similar error messages. (But i suspect the bs4 install has more issues so that's another question for later). </p>
  
  <p>Also sorry for any format issues with the code. Have tried my best to make it look like it is on the terminal.</p>
  
  <p>Thanks.</p>