Fix python 3.8 warnings in tests

[Pliner]

What do these changes do?

Fix deprecation warnings where a loop argument is passed to asyncio APIs. Continues #642

Are there changes in behavior for the user?

Nope

Checklist

• I think the code is well written
• Unit tests for the changes exist
• Documentation reflects the changes

[codecov]

Codecov Report

Merging #771 (97ace16) into master (9ba9b48) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #771   +/-   ##
=======================================
  Coverage   93.08%   93.08%           
=======================================
  Files          13       13           
  Lines        1532     1532           
  Branches      172      172           
=======================================
  Hits         1426     1426           
  Misses         78       78           
  Partials       28       28           

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