[skeleton] Update Debian packaging template
Here are multiple updates to the Debian packaging template provided when
running `cubicweb-ctl newcube`:
* Switch to source format 3.0 (quilt).
* Switch to debhelper 9.
* Switch to dh-python.
* Add Python 3 packages.
* Name the binary packages `python-cubicweb-CUBENAME` and
`python3-cubicweb-CUBENAME` instead of just `cubicweb-CUBENAME` (which is
still the source package).
* Populate Depends using dh_python{2,3} support for Python requirements.
* Run test suite at build time using pytest.
* Add autopkgtest to run test suite against the installed package.
* Bump Standards-Version to 4.3.0.