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.
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}