[devtools] Update skeleton's setup.py to install cube as a package Most of the prior logic of skeleton's setup.py gets dropped as installing a cube as a "classic package" (i.e. in site-packages) is just the default behavior of distutils. Also add a test checking installation of new cube. Related to #13001466.

../../doc/book usr/share/doc/cubicweb-documentation
../../doc/_build/html usr/share/doc/cubicweb-documentation