title: ImportError: cannot import name 'FormatControl' when trying to use pip
body: <p>I recently removed a python package (pymc3) and I think I probably messed it up and removed more than just that, because now I can't install anything with pip. When I try to use pip (or pip3) I get the following traceback and error message:</p>
  
  <pre><code> my_name:~ my_name$ pip3 install pymc3
  Traceback (most recent call last):
    File "/anaconda3/bin/pip3", line 6, in &lt;module&gt;
      from pip._internal.cli.main import main
    File "/anaconda3/lib/python3.6/site-packages/pip/_internal/__init__.py", line 42, in &lt;module&gt;
      from pip._internal import cmdoptions
    File "/anaconda3/lib/python3.6/site-packages/pip/_internal/cmdoptions.py", line 16, in &lt;module&gt;
      from pip._internal.index import (
  ImportError: cannot import name 'FormatControl'
  </code></pre>
  
  <p>Can anyone help me reconfigure my environment to get this working again? Thanks!</p>