Enable string normalization in Black

[soininen]

Just reformatting, no functional changes.

Re spine-tools/Spine-Toolbox#2684

Checklist before merging

• Code has been formatted by black
• Unit tests pass

[codecov]

Codecov Report

Attention: Patch coverage is 81.25000% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 61.66%. Comparing base (5790f2b) to head (fe14f57).

Files	Patch %	Lines
spine_items/tool/widgets/options_widgets.py	0.00%	2 Missing ⚠️
...ine_items/importer/widgets/import_editor_window.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           0.8-dev     #198      +/-   ##
===========================================
- Coverage    61.68%   61.66%   -0.03%     
===========================================
  Files          190      190              
  Lines        17656    17656              
  Branches      2890     2890              
===========================================
- Hits         10891    10887       -4     
- Misses        6183     6187       +4     
  Partials       582      582              

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