packaging: use /usr/bin/python3 shebang for scripts in Debian "hg" script is already correct because it is handled by setup.py but "hg-ssh" will be rewritten by dh_python into "/usr/bin/python" which is not wanted as we target Python 3. By passing --shebang=/usr/bin/python3 to dh_python3, we force shebangs to be set with this value.