Por que reinventar a roda?
dget http://httpredir.debian.org/debian/pool/main/p/python-numpy/python-numpy_1.12.0-2.dsc
cd python-numpy-1.12.0
Verifique PY3VERS em debian/rules
e depois reconstrua:
dch -n "Rebuild with Python 3.6."
dch -r ignored
dpkg-buildpackage -us -uc