title: install_requires in setup.py depending on installed Python version
body: <p>My setup.py looks something like this:</p>
  
  <pre><code>from distutils.core import setup
  
  setup(
      [...]
      install_requires=['gevent', 'ssl', 'configobj', 'simplejson', 'mechanize'],
      [...]
  )
  </code></pre>
  
  <p>Under Python 2.6 (or higher) the installation of the ssl module fails with:</p>
  
  <pre><code>ValueError: This extension should not be used with Python 2.6 or later (already built in), and has not been tested with Python 2.3.4 or earlier.
  </code></pre>
  
  <p>Is there a standard way to define dependencies only for specific python versions? Of course I could do it with <code>if float(sys.version[:3]) &lt; 2.6:</code> but maybe there is a better way to do it.</p>