Update the build system to use pybuild.

The new binary package with Python 3 support is named
python3-cubicweb-elasticsearch 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.

Both packages depends on the python{,3}-elasticsearch-dsl,
currently available at:
https://salsa.debian.org/lunar/elasticsearch-dsl-py

Fix MANIFEST.in to include missing test files.

Support for Python 3.8 is disabled as it is currently broken
in Debian unstable.