/usr/include/python3.5dm/modsupport.h is in libpython3.5-dbg 3.5.1-10.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/include/python3.5dm/../python3.5m/modsupport.h
/usr/include/python3.5dm/../python3.5m/modsupport.h