/usr/include/python3.5dm/sip.h is in python3-sip-dev 4.18.1+dfsg-2.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/include/python3.5dm/../python3.5m/sip.h
/usr/include/python3.5dm/../python3.5m/sip.h