title: All modules aren't available after upgrading to Python 3.8.5
body: <p>I just upgraded the Python version from 3.8.4 to 3.8.5
  And I faced a very major problem.
  All modules that I already installed aren't available with the following error message:</p>
  <pre><code>ModuleNotFoundError: No module named 'module name' 
  </code></pre>
  <p>So I tried to re-install the module with:</p>
  <pre><code>pip3 install module_name
  </code></pre>
  <p>And it returns an message:</p>
  <pre><code>Requirement already satisfied: module_name in /usr/local/lib/python3.8/site-packages (2.8.5)
  </code></pre>
  <p>OS : Mac os
  And I somehow figured it out that python libraries are saved on usr/lib/~
  and there are two versions, one python 3.7.3 and python 3.8.5</p>
  <p>And when I typed</p>
  <pre><code>python3 --version it printed out
  Python 3.7.3
  </code></pre>
  <p>However, when I tried to install throgh pip3 it checked the module on python 3.8.5 which I want to use.</p>
  <p>Therefore I think I need to change my default python3 version from python 3.7.3 to python 3.8.5.</p>
  <p>How to change the default python version to python 3.8.5?
  What's the problem here and how to fix it?</p>