Upgrading pip in virtualenvs managed by pipsi

I just spent a small amount of time wondering why pip would nag me to upgrade while using pipsi after I had already upgraded my system pip and the pip in the pipsi virtualenv. I finally realized that you need to upgrade pip in every single virtualenv (which is pretty obvious in hindsight, but well).

It would be nice if pipsi could offer a command to upgrade pip in a selected managed virtualenv, or in all managed virtualenvs at once.