Fix importing pivoted data with read_start_row

[soininen]

This PR fixes a special case where only the last component of import mappings is set to pivoted and read_start_row > 0.

Fixes spine-tools/Spine-Toolbox#2962

Checklist before merging

• [x] Documentation (also in Toolbox repo) is up-to-date
• [x] Release notes have been updated
• [x] Unit tests have been added/updated accordingly
• [x] Code has been formatted by black & isort
• [x] Unit tests pass

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 94.11765% with 1 line in your changes missing coverage. Please review.

Project coverage is 79.18%. Comparing base (9dd944e) to head (9d1c763). Report is 5 commits behind head on master.

Files with missing lines	Patch %	Lines
spinedb_api/export_mapping/settings.py	50.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #457 +/- ## ========================================== + Coverage 79.13% 79.18% +0.04% ========================================== Files 77 77 Lines 10102 10107 +5 Branches 1806 1810 +4 ========================================== + Hits 7994 8003 +9 + Misses 1760 1757 -3 + Partials 348 347 -1 ```

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