title: Python3.6 ImportError: cannot import name 'main' after upgrading pip from 8.1.1 to 19.0.1
body: <p>After I upgraded pip from 8.1.1 to 19.0.1 by running </p>
  
  <pre><code>pip install --upgrade pip
  </code></pre>
  
  <p>I tried to test the version of pip by running </p>
  
  <pre><code>pip -V
  </code></pre>
  
  <p>But I got the following error </p>
  
  <pre><code>Traceback (most recent call last):
    File "/usr/bin/pip3", line 9, in &lt;module&gt;
      from pip import main
  ImportError: cannot import name 'main'
  </code></pre>
  
  <p>I set the python3 and pip3 to be default by putting the following in <code>~/.bashrc</code></p>
  
  <pre><code>alias python=python3
  alias pip=pip3
  </code></pre>
  
  <p>My system is ubuntu 16.04</p>