Enable string normalization in Black

[soininen]

Just reformatting, no functional changes.

Re spine-tools/Spine-Toolbox#2684

Checklist before merging

• [x] Code has been formatted by black
• [x] Unit tests pass

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 90.19608% with 10 lines in your changes are missing coverage. Please review.

Project coverage is 78.30%. Comparing base (ca54884) to head (2cccc95).

Files	Patch %	Lines
...ons/39e860a11b05_add_alternatives_and_scenarios.py	71.42%	2 Missing :warning:
...b7c994c1c61_drop_object_and_relationship_tables.py	90.00%	2 Missing :warning:
spinedb_api/import_mapping/import_mapping.py	0.00%	2 Missing :warning:
spinedb_api/perfect_split.py	0.00%	2 Missing :warning:
spinedb_api/db_mapping.py	66.66%	1 Missing :warning:
spinedb_api/parameter_value.py	75.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## 0.8-dev #380 +/- ## ======================================== Coverage 78.30% 78.30% ======================================== Files 75 75 Lines 9495 9495 Branches 2011 2011 ======================================== Hits 7435 7435 Misses 1728 1728 Partials 332 332 ```

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