title: How to install / update package with pipenv without updating the rest of packages
body: <p>I use <strong>pipenv</strong> (version 2018.11.26) to manage dependencies in my project. Sometimes I want to add or update only one package and don't change versions of other packages. How can I achieve that? I've tried both</p>
  
  <pre><code>pipenv update --selective-upgrade requests
  </code></pre>
  
  <p>and</p>
  
  <pre><code>pipenv update --keep-outdated requests
  </code></pre>
  
  <p>but still versions of all the packages are updated during locking.</p>
  
  <p>Pipfile &amp; Pifile.lock:
  <a href="https://gist.github.com/jozo/d8351ed708e84c5ea0f69e82e585e5c6" rel="noreferrer">https://gist.github.com/jozo/d8351ed708e84c5ea0f69e82e585e5c6</a></p>