title: Can you set conditional dependencies for Python 2 and 3 in setuptools?
body: <p>When releasing a Python egg with support for both Python 2 and 3, can you specify dependencies that change depending on which version you're using? For example, if you use <code>dnspython</code> for Python 2, there is a Python 3 version that is called <code>dnspython3</code>. </p>
  
  <p>Can you write your <code>setuptools.setup()</code> function in such a way that your egg is useable to both versions if that is the only roadblock, i.e., if you have run <code>2to3</code> to ensure that the rest of your library is compatible with both versions.</p>
  
  <p>I have looked through these documents and can't seem to find the answer this question:</p>
  
  <ul>
  <li><a href="http://docs.python.org/3.3/howto/pyporting.html" rel="noreferrer">Porting Python 2 Code to Python 3</a></li>
  <li><a href="http://pythonhosted.org/distribute/setuptools.html#declaring-dependencies" rel="noreferrer">setuptools' Declaring Dependencies</a></li>
  <li><a href="http://about.travis-ci.org/docs/user/languages/python/" rel="noreferrer">Travis CI Python documentation</a></li>
  <li><a href="http://www.pip-installer.org/en/latest/cookbook.html#requirements-files" rel="noreferrer">pip Requirements Files</a> </li>
  </ul>