/usr/lib/python2.7/dist-packages/tracext/hg/backend.py is in trac-mercurial 1.0.0.3+hg8df754d9b36a-1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/python2.7/dist-packages/tracext/hg/../../../../../share/pyshared/tracext/hg/backend.py