Update elasticsearch-dsl requirement from ~=6.4 to >=6.4,<8.0

[dependabot-preview]

Updates the requirements on elasticsearch-dsl to permit the latest version.

Changelog

Sourced from elasticsearch-dsl's changelog.

7.1.0 (2019-10-23)

• Optimistic concurrent control for Document.delete
• Removing deprecated DocType
• Proper count caching for ES 7.x
• Support for multiplexer token filter
• Don't substitute for __ in FacetedSearch

7.0.0 (2019-04-26)

• Compatibility with Elasticsearch 7.x
• Document.save() now returns "created" or "updated"
• Dropped support for Python 2.6, 3.2, and 3.3
• When using fields the values are no longer merged into the body of the document and have to be accessed via .meta.fields only

6.4.0 (2019-04-26)

• Index.document now correctly sets the Document's _index only when using default index ([#1091](https://github.com/elasticsearch/elasticsearch-dsl-py/issues/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

6.3.1 (2018-12-05)

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

6.3.0 (2018-11-21)

• 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 implementated by @emarcey

6.2.1 (2018-07-03)

... (truncated)

Commits

• 24abee3 Version bump to 7.1.0
• 2c099d5 Adding RankFeature and RankFeature
• 38f2f5e adding distance feature query
• 4f63b2d Fixing the link to CLA
• e74ab2c Do not allow __ to dot expansion in FacetedSearch
• 89200d2 Adding parent aggregation
• d77ee3b Allow proper definitions of multiplexer and conditional token filters
• e1b4653 Explain example in docs
• ec9f9c0 Merge branch 'master' into connection_interface_usage
• 2b63582 Fixes count cache logic according to 7.x API
• Additional commits viewable in compare view

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[codecov]

Codecov Report

Merging #218 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #218   +/-   ##
=======================================
  Coverage   87.15%   87.15%           
=======================================
  Files          42       42           
  Lines        1892     1892           
=======================================
  Hits         1649     1649           
  Misses        243      243

Flag	Coverage Δ
#unittests	87.15% <ø> (ø)	⬆️

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[CasperWA]

From the changelog, it seems the major version change was due to added support for Elasticsearch of the same version (v7).
This seems in accordance with their documentation.

Hence, this change will allow Elasticsearch v6 and later, whereas before it was limited to Elasticsearch v6.
Finally, changing to elasticsearch-dsl~=7.1 would also be incorrect, since it would be the equivalent of elasticsearch-dsl>=7.1; <8.0. One could then do elasticsearch-dsl~=6, but one would have to add the condition of being minimum 6.4.0 anyway, so moving away from tilde notation here, makes the most sense.