title: Enforcing python version in setup.py
body: <p>Currently, we are setting\installing up some packages on system by mentioning their version and dependencies in <code>setup.py</code> under <code>install_requires</code> attribute. Our system requires Python 2.7. Sometimes, users are having multiple versions of Python on their systems, say 2.6.x and 2.7, some packages it says are available already but actually on the system available under 2.6 site packages list. Also some users have 2.6 only, how to enforce from <code>setup.py</code> or is there any other way to say to have only Python 2.7 and all packages which we want <code>setup.py</code> to update are for only 2.7. We require minimum 2.7 on the machine to run our code.</p>
  <p>Thanks!
  Santhosh</p>