title: Ignore argparse requirement in Python >2.7 only?
body: <p>Since argparse is in Python 2.7 but my 2.6 installs need it I have argparse in my requirements.txt. Is there a way to not install it when it is built into the installed Python already?</p>
  
  <p>I tried adding just 'argparse' with no version specified to my requirements.txt but pip still downloaded and installed argparse on my Mac running Python 2.7.1.</p>