Show Python 3.8 deprecation warning on Toolbox startup

[soininen]

At least we can tell users that we have warned them...

Re #2904

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 57.89474% with 8 lines in your changes missing coverage. Please review.

Project coverage is 66.95%. Comparing base (5cb26c0) to head (3fcba39).

Files	Patch %	Lines
spinetoolbox/ui_main.py	61.11%	7 Missing ⚠️
spinetoolbox/main.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2914      +/-   ##
==========================================
- Coverage   66.96%   66.95%   -0.02%     
==========================================
  Files         189      189              
  Lines       36878    36887       +9     
  Branches     6312     6314       +2     
==========================================
+ Hits        24695    24696       +1     
- Misses      10871    10879       +8     
  Partials     1312     1312              

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