Fix SQLAlchemy deprecation warnings

[soininen]

Fixed two minor warnings in the execution tests.

Re spine-tools/Spine-Database-API#477

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

All modified and coverable lines are covered by tests ✅

Project coverage is 67.21%. Comparing base (e6d97a9) to head (3d87852).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3049      +/-   ##
==========================================
- Coverage   67.22%   67.21%   -0.02%     
==========================================
  Files         194      194              
  Lines       37284    37284              
  Branches     4475     4475              
==========================================
- Hits        25063    25059       -4     
+ Misses      10953    10952       -1     
- Partials     1268     1273       +5     

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