Fix traceback caused by tool main file renaming

[PiispaH]

Fixes spine-tools/Spine-Toolbox#2890

Checklist before merging

• [x] Unit tests have been added/updated accordingly
• [x] Code has been formatted by black
• [x] Unit tests pass

[codecov[bot]]

Codecov Report

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

Project coverage is 62.37%. Comparing base (ee46d3d) to head (c8e7201).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #216 +/- ## ========================================== - Coverage 62.38% 62.37% -0.01% ========================================== Files 168 168 Lines 17832 17832 Branches 2979 2979 ========================================== - Hits 11124 11123 -1 - Misses 6103 6104 +1 Partials 605 605 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.