Update Debian packaging with support for Python 3
The new binary package with Python 3 support is named python3-cubicweb-jsonschema 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. We also take this opportunity to run the tests at build time. Support for Python 3.8 is disabled as it is currently broken in Debian unstable.