title: pip: ImportError: No module named retrying
body: <p>I'm trying to create a new environment and install various 3rd party packages on an Ubuntu machine. After having to install condo (for a <code>matplotlib</code> installation) and <code>gdal</code>, the <code>pip</code> function no longer works. Instead I receive:</p>
  
  <pre><code>Traceback (most recent call last):
    File "/media/imagery/ENVIRONMENTS/Project_1/bin/pip", line 7, in &lt;module&gt;
      from pip._internal import main
    File "/usr/lib/python2.7/dist-packages/pip/__init__.py", line 14, in &lt;module&gt;
      from pip.utils import get_installed_distributions, get_prog
    File "/usr/lib/python2.7/dist-packages/pip/utils/__init__.py", line 30, in &lt;module&gt;
      from pip._vendor.retrying import retry
  ImportError: No module named retrying
  </code></pre>
  
  <p>I've called pip both in and outside my environment, and still observe the same issue. Also I've tried a <code>pip install --upgrade pip</code> and still retrieve the same Traceback. Any ideas how to resolve this?</p>