'str' type in python2 is 'bytes' type in python3,
when use python3, we will get a prefix 'b':
sudo ip netns exec b'qrouter-39cc2b45-a27c-49c5-94a6-26443a49ac63' ip
neigh
--------------------------------------------------------------------------
*** Failed to run 'sudo ip netns exec
b'qrouter-39cc2b45-a27c-49c5-94a6-26443a49ac63' ip neigh': Command 'sudo
ip netns exec b'qrouter-39cc2b45-a27c-49c5-94a6-26443a49ac63' ip neigh'
returned non-zero exit status 1.
The message above is raised by running tools/worlddump.py with python3.
Change-Id: Ic254af86fa27729839f00c0ad4a5bbbc9e545a09