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 29.16667% with 51 lines in your changes are missing coverage. Please review.

Project coverage is 62.48%. Comparing base (2a045b5) to head (697fc85).

Files	Patch %	Lines
...ne/execution_managers/conda_kernel_spec_manager.py	0.00%	40 Missing ⚠️
...ine/execution_managers/conda_kernel_spec_runner.py	0.00%	5 Missing ⚠️
spine_engine/execution_managers/spine_repl.py	0.00%	2 Missing ⚠️
...pine_engine/server/persistent_execution_service.py	0.00%	2 Missing ⚠️
...execution_managers/persistent_execution_manager.py	75.00%	0 Missing and 1 partial ⚠️
spine_engine/server/util/server_message.py	90.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           0.8-dev     #134   +/-   ##
========================================
  Coverage    62.48%   62.48%           
========================================
  Files           46       46           
  Lines         3916     3916           
  Branches       763      763           
========================================
  Hits          2447     2447           
  Misses        1312     1312           
  Partials       157      157           

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