requirements: update craft-parts to 1.21.1

[cmatsuoka]

Craft-parts 1.21.1 reverts changes in pull and build subdirs.

• 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 #4208 (c28817e) into main (a9025c0) will not change coverage.
The diff coverage is n/a.

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@@           Coverage Diff           @@
##             main    #4208   +/-   ##
=======================================
  Coverage   93.90%   93.90%           
=======================================
  Files         569      569           
  Lines       43746    43746           
=======================================
  Hits        41078    41078           
  Misses       2668     2668           

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