title: Install-time dependencies in requirements.txt
body: <p>I'm using tox to prepare venv and run unit tests and my application needs openopt library which in turn imports <code>numpy.distutils.core</code> in its setup.py.</p>
  
  <p>No matter how I order numpy and openopt in my requirements.txt I can't ensure numpy is installed before setup.py from openopt is executed and exit with <code>ImportError: No module named numpy.distutils.core</code></p>
  
  <p>How can I fix that? For development I can add numpy to requirements.txt, run tox, add openopt and run tox again but it's not production-ready setup.</p>