[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

Starting from ES6, `_type` metadata is deprecated,
cf. https://www.elastic.co/guide/en/elasticsearch/reference/current/removal-of-types.html

If this is not done, elasticsearch.scan method retrieves the old index values
so wrong values will be copied while synchronizing different indexes.

We probably should do it by default in `indexer.es_index` method.
We probably should do it in `indexer.es_delete` method.

see also https://www.elastic.co/guide/en/elasticsearch/reference/current/docs-refresh.html

related to #17090555

When indexing an entity type, don't generate a single
and possibly huge rset with all entities of that type.
Instead, fetch entities chunk by chunk and free caches
between each round.

closes #17090555

cubicweb_elasticsearch/migration/* was not included in source package.