Update to async/await syntax

[benhowes]

This PR simply upgrades to modern syntax, which is deprecated as of python3.8 (and available as of 3.6)

• @asyncio.coroutine -> async def
• yield from -> await

Neatly described in this SO answer https://stackoverflow.com/a/44273861

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[sethmlarson]

Thank you for the interest!

This work is already being done as a part of migrating AsyncElasticsearch to the main elasticsearch package: elastic/elasticsearch-py#1232
Will be landing soon so I'll close this PR as duplicate.

[benhowes]

Ah perfect, thanks!