title: pip3 -V should print pip3 version but returns No module named 'pip' in Ubuntu 16.04
body: <p>I am using Python 3.7.2 in Ubuntu 16.04. I installed the <em>python3-pip</em> package from the default Ubuntu repositories, and it is reported by apt that I have installed python3-pip 8.1.1-2ubuntu0.4 (version 8.1.1), but pip3 does not work properly. <code>pip3 -V</code> should print the pip3 version, but it returns <code>No module named 'pip'</code> instead. </p>
  
  <pre class="lang-none prettyprint-override"><code>root@VM-0-8-ubuntu:/usr/bin# python -V
  Python 3.7.2
  root@VM-0-8-ubuntu:/usr/bin# pip3 -V
  Traceback (most recent call last):
    File "/usr/bin/pip3", line 9, in &lt;module&gt;
      from pip import main
  ModuleNotFoundError: No module named 'pip'
  root@VM-0-8-ubuntu:/usr/bin# sudo apt-get install python3-pip
  Reading package lists... Done
  Building dependency tree       
  Reading state information... Done
  python3-pip is already the newest version (8.1.1-2ubuntu0.4).
  0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.
  root@VM-0-8-ubuntu:/usr/bin# pip3 -V
  Traceback (most recent call last):
    File "/usr/bin/pip3", line 9, in &lt;module&gt;
      from pip import main
  ModuleNotFoundError: No module named 'pip'
  root@VM-0-8-ubuntu:/usr/bin# sudo apt updaate
  E: Invalid operation updaate
  root@VM-0-8-ubuntu:/usr/bin# sudo apt update
  Hit:1 http://mirrors.tencentyun.com/ubuntu xenial InRelease
  Hit:2 http://mirrors.tencentyun.com/ubuntu xenial-security InRelease                 
  Ign:3 http://mirrors.aliyun.com/ubuntu trusty InRelease                              
  Hit:4 http://mirrors.tencentyun.com/ubuntu xenial-updates InRelease     
  Hit:5 http://mirrors.aliyun.com/ubuntu trusty-security InRelease        
  Hit:6 http://mirrors.aliyun.com/ubuntu trusty-updates InRelease                   
  Hit:7 http://mirrors.aliyun.com/ubuntu trusty-proposed InRelease                  
  Hit:8 http://mirrors.aliyun.com/ubuntu trusty-backports InRelease                 
  Hit:9 http://mirrors.aliyun.com/ubuntu trusty Release                             
  Reading package lists... Done 
  Building dependency tree       
  Reading state information... Done
  All packages are up to date.
  root@VM-0-8-ubuntu:/usr/bin# sudo apt upgrade
  Reading package lists... Done
  Building dependency tree       
  Reading state information... Done
  Calculating upgrade... Done
  0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.
  root@VM-0-8-ubuntu:/usr/bin# sudo apt-get install python3-pip
  Reading package lists... Done
  Building dependency tree       
  Reading state information... Done
  python3-pip is already the newest version (8.1.1-2ubuntu0.4).
  0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.
  root@VM-0-8-ubuntu:/usr/bin# pip3 -V
  Traceback (most recent call last):
    File "/usr/bin/pip3", line 9, in &lt;module&gt;
      from pip import main
  ModuleNotFoundError: No module named 'pip'
  root@VM-0-8-ubuntu:/usr/bin#
  </code></pre>