Specify Python Version for Virtualenv in Requirements.txt
it would be really convenient not to have to tell every new person
  joining the team how to set up their virtualenv

Just add it to the normal set of instructions you give new members when the join; right in the same place when you tell them about the internal documentation wiki, the password to the wifi and the phone number to the sandwich delivery shop.
It will be extremely uncovenient to not have to tell people and have them figure it out themselves; the first time they submit something that uses collections.Counter only to find out it broke the build because the server doesn't have 2.7.x
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? 
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.