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.

The httplib module has been renamed to http.client in Python 3.
So let's try to import HTTPConnection from both.

- Fix errors:
  - TypeError: 'float' object cannot be interpreted as an integer

- Fix deprecation warnings:
  - DeprecationWarning: Please use assertEqual instead
  - DeprecationWarning: Please use assertTrue instead

[dependency] pin python-elasticsearch and python-elasticsearch-dsl versions as elasticsearch 7 has been released

previously bool value was 0 or 1 it seems that elasticsearch 6 will not cast
this value anymore

this magic field does not exist anymore with elasticsearch 6 use computed field which it is named alltext by convention

this magic field does not exist anymore with elasticsearch 6
use computed field which it is named `alltext` by convention

[elasticserach] elasticsearch.client.Elasticsearch.index expect a string as value of `refresh` param