bashrc: fix all entries in PYTHONPATH being duplicated

My previous attempt at not duplicating paths in PATH and PYTHONPATH -
a function I'd called `path_append` - was not actually preventing pre-
existing paths being added. I've now copied a function called `pathmunge`
from /etc/profile which does this job properly (I've got almost-copies
for PATH and PYTHONPATH separately). This is now doing the right thing.