Elasticsearch-dsl

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* ``Index.document`` now correctly sets the ``Document``'s ``_index`` only when
using default index (``1091``)
* ``Document`` inheritance allows overriding ``Object`` and ``Nested`` field metadata like ``dynamic``
* adding ``auto_date_histogram`` aggregation
* Do not change data in place when (de)serializing

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* ``Analyzer.simulate`` now supports built-in analyzers
* proper (de)serialization of the ``Range`` wrapper
* Added ``search_analyzer`` to ``Completion`` field

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* Fixed logic around defining a different ``doc_type`` name.
* Added ``retry_on_conflict`` parameter to ``Document.update``.
* fields defined on an index are now used to (de)serialize the data even when
not defined on a ``Document``
* Allow ``Index.analyzer`` to construct the analyzer
* Detect conflict in analyzer definitions when calling ``Index.analyzer``
* Detect conflicting mappings when creating an index
* Add ``simulate`` method to ``analyzer`` object to test the analyzer using the
``_analyze`` API.
* Add ``script`` and ``script_id`` options to ``Document.update``
* ``Facet`` can now use other metric than ``doc_count``
* ``Range`` objects to help with storing and working with ``_range`` fields
* Improved behavior of ``Index.save`` where it does a better job when index
already exists
* Composite aggregations now correctly support multiple ``sources`` aggs
* ``UpdateByQuery`` implemented by emarcey

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* allow users to redefine ``doc_type`` in ``Index`` (``929``)
* include ``DocType`` in ``elasticsearch_dsl`` module directly (``930``)

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**Backwards incompatible change** - ``DocType`` refactoring.

In ``6.2.0`` we refactored the ``DocType`` class and renamed it to
``Document``. The primary motivation for this was the support for types being
dropped from elasticsearch itself in ``7.x`` - we needed to somehow link the
``Index`` and ``Document`` classes. To do this we split the options that were
previously defined in the ``class Meta`` between it and newly introduced
``class Index``. The split is that all options that were tied to mappings (like
setting ``dynamic = MetaField('strict')``) remain in ``class Meta`` and all
options for index definition (like ``settings``, ``name``, or ``aliases``) got
moved to the new ``class Index``.

You can see some examples of the new functionality in the ``examples``
directory. Documentation has been updated to reflect the new API.

``DocType`` is now just an alias for ``Document`` which will be removed in
``7.x``. It does, however, work in the new way which is not fully backwards
compatible.

* ``Percolator`` field now expects ``Query`` objects as values
* you can no longer access meta fields on a ``Document`` instance by specifying
``._id`` or similar. Instead all access needs to happen via the ``.meta``
attribute.
* Implemented ``NestedFacet`` for ``FacetedSearch``. This brought a need to
slightly change the semantics of ``Facet.get_values`` which now expects the
whole data dict for the aggregation, not just the ``buckets``. This is
a backwards incompatible change for custom aggregations that redefine that
method.
* ``Document.update`` now supports ``refresh`` kwarg
* ``DslBase._clone`` now produces a shallow copy, this means that modifying an
existing query can have effects on existing ``Search`` objects.
* Empty ``Search`` no longer defaults to ``match_all`` query and instead leaves
the ``query`` key empty. This is backwards incompatible when using
``suggest``.

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* Removed ``String`` field.
* Fixed issue with ``Object``/``Nested`` deserialization