Add support for the "boxplot" aggregation

[hugorodgerbrown]

This commit adds a new Agg subclass called BoxPlot, which passes the name "boxplot" as the aggregation type.

See https://www.elastic.co/guide/en/elasticsearch/reference/7.x/search-aggregations-metrics-boxplot-aggregation.html#_syntax

Fixes #1411

[cla-checker-service]

💚 CLA has been signed

[hugorodgerbrown]

CI is failing on Black linting - I haven't edited any of the files that are failing, but there has been an update to Black since the last successful build, which may explain it.

[hugorodgerbrown]

Confirmed - have re-run Black using the latest version.

[hugorodgerbrown]

I have also removed the "standard" token filter from the test_elasticsearch_dsl.test_integration.test_document.snowball analyzer, as it was causing a test failure, and has been removed from ES7.

[hugorodgerbrown]

Fixed isses in elasticsearch_dsl - CI now failing on building coverage on the nightly build:

  running install_scripts
  Traceback (most recent call last):
    File "<string>", line 1, in <module>
    File "/tmp/pip-wheel-ze049w3s/coverage/setup.py", line 213, in <module>
      main()
    File "/tmp/pip-wheel-ze049w3s/coverage/setup.py", line 203, in main
      setup(**setup_args)
    File "/home/travis/virtualenv/python3.10-dev/lib/python3.10/site-packages/setuptools/__init__.py", line 163, in setup
      return distutils.core.setup(**attrs)
    File "/opt/python/3.10-dev/lib/python3.10/distutils/core.py", line 148, in setup
      dist.run_commands()
    File "/opt/python/3.10-dev/lib/python3.10/distutils/dist.py", line 966, in run_commands
      self.run_command(cmd)
    File "/opt/python/3.10-dev/lib/python3.10/distutils/dist.py", line 985, in run_command
      cmd_obj.run()
    File "/home/travis/virtualenv/python3.10-dev/lib/python3.10/site-packages/wheel/bdist_wheel.py", line 328, in run
      impl_tag, abi_tag, plat_tag = self.get_tag()
    File "/home/travis/virtualenv/python3.10-dev/lib/python3.10/site-packages/wheel/bdist_wheel.py", line 278, in get_tag
      assert tag in supported_tags, "would build wheel with unsupported tag {}".format(tag)
  AssertionError: would build wheel with unsupported tag ('cp310', 'cp310', 'linux_x86_64')

[hugorodgerbrown]

I have allowed the "nightly" python build to fail on Travis.

[sethmlarson]

@hugorodgerbrown Thanks for these changes. Could you add the changes to CI / tests to their own PR and keep this only BoxPlot agg so I can backport automatically? :)

[sethmlarson]

Closing this in favor of #1416