Fix not able to open Toolbox due to Traceback from imports

[PiispaH]

Fixed imports

Fixes spine-tools/Spine-Toolbox#2427

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-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (e4f3fe6) 60.08% compared to head (f9e436e) 60.08%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## 0.8-dev #170 +/- ## =========================================== - Coverage 60.08% 60.08% -0.01% =========================================== Files 194 194 Lines 17651 17650 -1 Branches 2872 2872 =========================================== - Hits 10606 10605 -1 Misses 6477 6477 Partials 568 568 ```

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