Issue 2223 mapping name importer source

[PiispaH]

https://github.com/spine-tools/Spine-Database-API/pull/423

Re spine-tools/Spine-Toolbox#2223

Checklist before merging

• [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
• [x] Change back to default requirements

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 9.09091% with 10 lines in your changes missing coverage. Please review.

Project coverage is 62.33%. Comparing base (752e7c7) to head (64c91d3).

:exclamation: Current head 64c91d3 differs from pull request most recent head a867b89

Please upload reports for the commit a867b89 to get more accurate results.

Files	Patch %	Lines
spine_items/importer/flattened_mappings.py	0.00%	4 Missing and 2 partials :warning:
spine_items/importer/mvcmodels/mappings_model.py	0.00%	3 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #214 +/- ## ========================================== - Coverage 62.36% 62.33% -0.04% ========================================== Files 168 168 Lines 17741 17750 +9 Branches 2951 2955 +4 ========================================== - Hits 11065 11064 -1 - Misses 6074 6080 +6 - Partials 602 606 +4 ```

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