/usr/lib/python2.7/dist-packages/RestrictedPython/MutatingWalker.py is in python-restrictedpython 3.6.0-0ubuntu4.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/python2.7/dist-packages/RestrictedPython/../../../../share/pyshared/RestrictedPython/MutatingWalker.py