More SQLAlchemy compatibility

[soininen]

With this, spinedb_api should be ready for SQLAlchemy 2.0.

Resolves #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 77.77778% with 2 lines in your changes missing coverage. Please review.

Project coverage is 81.92%. Comparing base (308c085) to head (934bb57).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
...edb_api/spine_io/importers/sqlalchemy_connector.py	0.00%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #488   +/-   ##
=======================================
  Coverage   81.92%   81.92%           
=======================================
  Files          78       78           
  Lines       10306    10306           
  Branches     1550     1550           
=======================================
  Hits         8443     8443           
  Misses       1493     1493           
  Partials      370      370           

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