title: weasyprint doesn't apply css style in output file
body: <p>I'm trying to output a <code>pdf</code> from an <code>html</code> page using a <code>python</code> script with <code>weasyprint</code> library. I got the output as a <code>pdf</code> file, but the <code>pdf</code> page is not styled however I'm using bootstrap css style. This is a snippet of my code:</p>
  <pre><code>HTML('./result.html').write_pdf('./result.pdf', stylesheets=[CSS(filename='./bootstrap.css')])
  </code></pre>
  <p>result.html file:</p>
  <pre><code>&lt;html&gt;
  &lt;head&gt;
      &lt;link rel=&quot;stylesheet&quot; href=&quot;bootstrap.css&quot;&gt;
  &lt;/head&gt;
  &lt;body&gt;
  &lt;h1 class='text-primary'&gt;Title&lt;/h1&gt;
  &lt;/body&gt;
  &lt;/html&gt;
  </code></pre>
  <p>WeasyPrint version: 0.40</p>