requirements: update craft-parts to 1.19.6

[cmatsuoka]

Craft Parts 1.19.6 reverts subdir changes in pull and build steps.

• Have you followed the guidelines for contributing?
• Have you signed the CLA?
• Have you successfully run make lint?
• Have you successfully run pytest tests/unit?

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[codecov-commenter]

Codecov Report

Merging #4207 (cd486f9) into hotfix/7.4 (389462c) will not change coverage.
The diff coverage is n/a.

❗ Current head cd486f9 differs from pull request most recent head 744bf86. Consider uploading reports for the commit 744bf86 to get more accurate results

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@@             Coverage Diff             @@
##           hotfix/7.4    #4207   +/-   ##
===========================================
  Coverage       94.85%   94.85%           
===========================================
  Files             648      648           
  Lines           54353    54353           
===========================================
  Hits            51554    51554           
  Misses           2799     2799           

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