Fix SQLAlchemy warnings

[soininen]

This PR fixes SQLAlchemy warnings including all deprecation warnings from unit tests. No functional changes intended. This should bring us a step closer to SQLAlchemy 2.0.

Re #477

Checklist before merging

• Documentation (also in Toolbox repo) 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 69.76744% with 13 lines in your changes missing coverage. Please review.

Project coverage is 81.55%. Comparing base (cefebcb) to head (f3a182f).
Report is 5 commits behind head on master.

Files with missing lines	Patch %	Lines
spinedb_api/db_mapping.py	53.84%	9 Missing and 3 partials ⚠️
spinedb_api/helpers.py	90.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #486   +/-   ##
=======================================
  Coverage   81.55%   81.55%           
=======================================
  Files          78       78           
  Lines       10308    10309    +1     
  Branches     1551     1551           
=======================================
+ Hits         8407     8408    +1     
  Misses       1538     1538           
  Partials      363      363           

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