s4-python Don't force "bin" into the python sys.path

s4-python Don't force "bin" into the python sys.path

This patch uses the fact that dyn_PYTHONDIR and dyn_PYTHONARCHDIR is
now set properly at all times, incluidng for installed and
build-directory binaries.  So we don't need to make assumptions about
bin being magic any more.

Andrew Bartlett