PyBison is a YACC (Yet Another Compiler Compiler) style parser. PyBison was written by Scott Hassan in 1997 and almost disappeared.
There are scattered references to it all over the web, which all seem to point to http://coho.stanford.edu/~hassan/Python/pybison.tar.gz, which forwards to the dead link http://dotfunk.com/hassan/homepage/Python
ActiveState has a copy (version 0.1.0.0 as of 2005-08-19):
More recent copies can be found at (verion 0.1.8 as of 2005-08-19):