Drop imports from deprecated distutils

[soininen]

Re spine-tools/Spine-Toolbox#2522

Checklist before merging

• Code has been formatted by black
• Unit tests pass

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 61.68%. Comparing base (00b93ca) to head (99ae068).

Files	Patch %	Lines
spine_items/importer/flattened_mappings.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           0.8-dev     #199      +/-   ##
===========================================
- Coverage    61.68%   61.68%   -0.01%     
===========================================
  Files          190      190              
  Lines        17656    17655       -1     
  Branches      2890     2890              
===========================================
- Hits         10891    10890       -1     
  Misses        6183     6183              
  Partials       582      582              

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