Remove pyatom

Pyatom dependency is simply an extracted version of Werkzeug's atom feed (http://werkzeug.pocoo.org/docs/contrib/atom/). Since Werkzeug is already a dependency, no need for the duplication. From [pyatom](https://pypi.python.org/pypi/pyatom):

> Extracted from werkzeug.contrib.atom and created as a standalone package

Therefore removed from requirements.txt, and updated flamelinks.py as appropriate.