/usr/share/gdb/auto-load/usr/lib/x86_64-linux-gnu/libpython3.5m.so.1.0-gdb.py is in python3.5-dbg 3.5.3-1+deb9u1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/gdb/auto-load/usr/lib/x86_64-linux-gnu/../../bin/python3.5m-gdb.py