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