title: Upgrade python in a virtualenv
body: <p>Is there a way to upgrade the version of python used in a virtualenv (e.g. if a bugfix release comes out)?</p>
  
  <p>I could <code>pip freeze --local &gt; requirements.txt</code>, then remove the directory and <code>pip install -r requirements.txt</code>, but this requires a lot of reinstallation of large libraries, for instance, <code>numpy</code>, which I use a lot. </p>
  
  <p>I can see this is an advantage when upgrading from, e.g., 2.6 -> 2.7, but what about 2.7.x -> 2.7.y?</p>