Update _restclient asyncio testing syntax

[jarq6c]

Updates (yet again) the event_loop syntax in the _restclient tests.

https://pytest-asyncio.readthedocs.io/en/stable/how-to-guides/migrate_from_0_21.html

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[jarq6c]

FYI: @aaraney I'll merge this later this week.

[aaraney]

Thanks for being on top of and handling this grunt work, @jarq6c!