[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.

version=3
opts=uversionmangle=s/(rc|a|b|c)/~$1/ \
https://pypi.debian.net/cubicweb/cubicweb-(.+)\.(?:zip|tgz|tbz|txz|(?:tar\.(?:gz|bz2|xz)))