Add Elasticsearch-DSL code

[miguelgrinberg]

This change adds the Elasticsearch-DSL code. Note that the documentation will be merged in a separate PR.

• The imported source tree has the same structure as in the elasticsearch_dsl package, except that its root is now elasticsearch.dsl. Users can replace the _ with a . in all imports.
• I had to make minor changes to some of the DSL's test fixtures, because this client uses function scope for the client fixtures and DSL used session scope. I have switched all the DSL fixtures to function scope.
• I had to make the DSL client fixtures work with https:// Elasticsearch endpoints, since that is what we are using in integration test runs.
• The unasync of the DSL classes remains in a separate run-unasync-dsl.py script. We can consider merging the two.
• The generator.py script that generates the query, aggregation and response source files was moved to this project as generator-dsl.py, along with its Jinja templates. We can consider moving this to the Python client generator project.

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[miguelgrinberg]

@pquentin The builds are currently not generating code coverage data, and I haven't been able to figure out why. And I get coverage data when I run the tests locally. I'll continue looking for this, but if you have any ideas why this is happening let me know. Other than this the PR is ready for a review.

[pquentin]

• The unasync of the DSL classes remains in a separate run-unasync-dsl.py script. We can consider merging the two.
• The generator.py script that generates the query, aggregation and response source files was moved to this project as generator-dsl.py, along with its Jinja templates. We can consider moving this to the Python client generator project.

Yes, please, as a follow-up.

@pquentin The builds are currently not generating code coverage data, and I haven't been able to figure out why. And I get coverage data when I run the tests locally. I'll continue looking for this, but if you have any ideas why this is happening let me know. Other than this the PR is ready for a review.

I haven't looked in detail yet, but the current build has 13% coverage, and we don't use coverage for anything meaningful today. However it would be nice to fix this issue as I'm seeing 0% coverage here. (In the future, we can exclude generated code to make coverage more useful.)

[miguelgrinberg]

@pquentin The last commit addresses your feedback. One thing I want to direct your attention to, is the handling of the user agent, which I only noticed now. The connection class used to add an elasticsearch-dsl-py/$VERSION user agent. I changed that to use elasticsearch-py/$VERSION, but I can't decide if this is a good idea or we should better leave it with the DSL user agent in case we want to see how many people use the DSL classes going forward. Let me know what your thoughts are on this.

[pquentin]

Thanks! Sorry, I did not realize my mypy suggestion would bring so much changes.

[pquentin]

Thanks! LGTM. 🚀