Fix RunTimeWarning

[ptsavol]

Set default event loop policy for asyncio.

Fixes #3027

Checklist before merging

• Documentation is up-to-date
• Release notes have been updated
• Unit tests have been added/updated accordingly
• Code has been formatted by black & isort
• Unit tests pass

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.

Project coverage is 67.12%. Comparing base (7a70978) to head (5f92931).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
spinetoolbox/main.py	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3028      +/-   ##
==========================================
- Coverage   67.14%   67.12%   -0.02%     
==========================================
  Files         193      193              
  Lines       37079    37082       +3     
  Branches     4446     4447       +1     
==========================================
- Hits        24896    24893       -3     
- Misses      10925    10931       +6     
  Partials     1258     1258              

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