title: Specify Python Version for Virtualenv in Requirements.txt
body: <p>I'm using virtualenv to develop a django application with a team. The server we're deploying on is running python 2.6, but the default for our machines is 2.7.3. Is there any way to specify python version in the requirements.txt file, or something similar, within the code base? </p>
  
  <p>I know requirements.txt is a pip thing, and python version is a virtualenv thing, but it would be really convenient not to have to tell every new person joining the team how to set up their virtualenv.</p>