Only use existing tools when migrating, not when committing

[manuelma]

Fixes #355

Checklist before merging

• [x] Documentation (also in Toolbox repo) is up-to-date
• [x] Release notes in Toolbox repo have been updated
• [x] Unit tests have been added/updated accordingly
• [x] Code has been formatted by black
• [x] Unit tests pass

[codecov[bot]]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (d806612) 77.36% compared to head (38b44a4) 77.33%.

Files	Patch %	Lines
spinedb_api/compatibility.py	81.81%	4 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## 0.8-dev #367 +/- ## =========================================== - Coverage 77.36% 77.33% -0.03% =========================================== Files 75 75 Lines 9406 9412 +6 Branches 1989 1993 +4 =========================================== + Hits 7277 7279 +2 - Misses 1800 1804 +4 Partials 329 329 ```

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