Test for, and suppress, additional get_event_loop() DeprecationWarnings

[SyntaxColoring]

This attempts to do what's described in #757 (comment), to solve continued instances of issue #757:

• Add tests to catch any lingering asyncio.get_event_loop() DeprecationWarnings.
  • The existing tests are already catching a bunch, but only when they're run under specific Python patch versions. So the test part of this PR just configures tox and GitHub Actions to do that.
  • I could only reproduce the warnings on specific Python 3.10.x patch versions, and I couldn't easily get CI to use those versions, so we don't have tests for this.
• Suppress those warnings.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.59%. Comparing base (dfe6e7c) to head (90e6137).
Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #817   +/-   ##
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  Coverage   95.59%   95.59%           
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  Files           2        2           
  Lines         477      477           
  Branches       96       95    -1     
=======================================
  Hits          456      456           
  Misses         12       12           
  Partials        9        9           

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[seifertm]

Thanks a lot for looking into this!

[SyntaxColoring]

@seifertm Ping—do the latest changes address your feedback?