title: In Python, `pip -r requirements.txt` doesn't install packages *recursively*?
body: <p>I'm using Python 3.5.1 and pip 7.1.2:</p>
  
  <pre><code>pip3 --version
  pip 7.1.2 from /Library/Frameworks/Python.framework/Versions/3.5/lib/python3.5/site-packages (python 3.5)
  </code></pre>
  
  <p>In my requirements.txt, I write:</p>
  
  <pre><code>pysam&gt;=0.9.0
  </code></pre>
  
  <p>Then I use <code>pip3</code> to install this requirements.txt file like this:</p>
  
  <pre><code>pip3 install -U -r requirements.txt
  </code></pre>
  
  <p>This <code>pysam</code> has its own dependencies: <code>cython</code>, which can be seen at: <a href="https://github.com/pysam-developers/pysam/blob/master/requirements.txt" rel="noreferrer">https://github.com/pysam-developers/pysam/blob/master/requirements.txt</a></p>
  
  <p>However, using <code>pip3 install -U -r requirements.txt</code> seems not to install <code>pysam</code>'s dependency recursively, it will throw an expection that <code>ValueError: no cython installed</code>.</p>
  
  <p>Does anyone have ideas about why <code>requirements.txt</code> are not installed <em>recursively</em>?</p>