title: Python 'requirements.txt' file in package
body: <p><strong>SITUATION:</strong></p>
  <p>I have created a python package where I use the following libraries:</p>
  <ol>
  <li>matplotlib</li>
  <li>regex</li>
  <li>statistics</li>
  <li>os</li>
  <li>unittest</li>
  <li>coverage</li>
  </ol>
  <p>my problem is that when I do <code>pip freeze</code>, the result only returns
  versions values for</p>
  <ol>
  <li>matplotlib==3.2.1</li>
  <li>regex==2020.11.13</li>
  <li>statistics==1.0.3.5</li>
  <li>coverage==5.3.1</li>
  </ol>
  <p>I have read some sites where they say that this is because, for example,
  os and unittest comes already installed with Python3.X.</p>
  <p><strong>QUESTION:</strong></p>
  <ol>
  <li>Should I include 'os', 'unittest' in 'requirements.txt'?</li>
  <li>If so, which is the version I should write?</li>
  </ol>