Update Debian packaging with support for Python 3
Update the build system to use pybuild. The new binary package with Python 3 support is named python3-cubicweb-blog according to current standards. Python 2 package is also updated to use the proper namespace and a transitional package has been introduced to handle the migration. Fix MANIFEST.in to include a missing image, extra files to run the tests and to not ship packaging files. Support for Python 3.8 is disabled as it is currently broken in Debian unstable.