Distrowatch is a simple tool to see how out of date your pinned requirements.txt packages are, as compared to pypi. All packages are saved to a json file, which can be served however you wish (I prefer mine medium-rare).
This is intended to be as pure python as possible, so that it's agnostic with regards to platform. The only outside library is pip, which you should have anyway.
Run get_versions.py. I prefer to run this through a cronjob.
This script uses the json library, so it requires Python 2.6 or later.