netns: [critical] fix python3 / ctypes issue

W/o this patch, netns tries to operate on / dir instead of netns dir, since
ctypes handles default strings in Python2 and Python3 differently.

Bug-Url: https://github.com/svinota/pyroute2/issues/84
tests: cover netns

Bug-Url: https://github.com/svinota/pyroute2/issues/84