2.0/3.0 release changes

[max-moser]

This issue tracks the changes made in PRs leading up to v2.0.

Changes from https://github.com/inveniosoftware/invenio-stats/pull/118

- Upgrade to ``invenio-search`` 2.x
- Drop support for Elasticsearch 2, 5, and 6
- Add support for OpenSearch 1 and 2
- Drop support for Python 2.7 and 3.6
- Remove function ``invenio_stats.utils:get_doctype``
- Update dependencies

[max-moser]

Changes from https://github.com/inveniosoftware/invenio-stats/pull/126:

- Fix ``validate_arguments`` for query classes
- Add ``build_event_emitter`` function for creating an ``EventEmitter`` but not registering it as a signal handler
- Add ``ext.get_event_emitter(name)``` function for caching built ``EventEmitter`` objects per name

[max-moser]

Changes from https://github.com/inveniosoftware/invenio-stats/pull/131:

- Replace elasticsearch-specific terminology

[max-moser]

:warning: this PR isn't merged Changes from https://github.com/inveniosoftware/invenio-stats/pull/130:

- Allow configuration of simple bucket aggregations