cubicweb #10692416 [skeleton] do not import the __pkginfo__.py module to make cubes installable via setuptools [validation pending]

[skeleton] do not import the __pkginfo__.py module to make cubes installable via setuptools

When using a simple "import" statement, running 'python setup.py install' for a cube fails in a fresh virtualenv due to the fact the install of the dependencies (here cubicweb iteself) is done by the python process doing the cube's installation, so the __pkginfo__ module from the cube is already loaded (thus used) when installing cubicweb...

So we apply the third technique from:

https://packaging.python.org/en/latest/single_source_version.html
prioritynormal
typebug
done in3.23.0
load left0.000
closed by<not specified>