/usr/lib/python3.5/config-3.5dm-x86_64-linux-gnu/libpython3.5dm.so is in libpython3.5-dbg 3.5.3-1+deb9u1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/python3.5/config-3.5dm-x86_64-linux-gnu/../../x86_64-linux-gnu/libpython3.5dm.so