title: setup.py: Depend on CSS and JS packages
body: <p>I have <a href="https://github.com/nschloe/tuna" rel="nofollow noreferrer">a Python project</a> which displays some information in the browser and uses <a href="https://getbootstrap.com/" rel="nofollow noreferrer">bootstrap</a> and <a href="https://d3js.org/" rel="nofollow noreferrer">D3</a> to do so. Right now, I'm simply including <code>bootstrap.min.css</code> and <code>d3.v5.min.js</code> in the source code repository, add them to the <code>package_data</code> in <code>setup.py</code>, and update them as new versions are released. This is rather ugly of course.</p>
  
  <p>I'd like to specify bootstrap and D3 as a dependency in <code>setup.py</code>; any hint on how this could be possible?</p>