title: requirements.txt depending on python version
body: <p>I'm trying to port a python2 package to python3 (not my own) using six so that it's compatible with both.  However one of the packages listed in requirements.txt is now included in the python3 stdlib and the pypi version doesn't work in python3 so I want to conditionally exclude it.  Doing this in setup.py is easy, I can just do something like:</p>
  
  <pre><code>if sys.version_info[0] == 2:
      requirements += py2_requirements
  else:
      requirements += py3_requirements
  </code></pre>
  
  <p>But I would like requirements.txt to reflect the correct list too.  I can't find anything on this in the pip documentation. so does anyone know how to do it, or if it is even possible?</p>