Trio support

If/when elastic-transport-python and elasticsearch-py support Trio as an alternative to asyncio, then we could add support to Trio to the DSL client as well.

One of the first steps would be to switch pytest-asyncio's mode from auto to strict: https://github.com/elastic/elasticsearch-dsl-py/pull/1714/files#r1527918832.