🩹 change PyPI upload option to kebab-case

[burgholzer]

Description

https://github.com/pypa/gh-action-pypi-publish changed the naming convention for action parameters from snake_case to kebab-case. While the old syntax will be supported until the next major version increase, a warning is admitted as of the latest version. This PR changes the skip_existing option to skip-existing to reflect the above change and silence the warning.

Checklist:

• [x] The pull request only contains commits that are related to it.
• [x] I have added appropriate tests and documentation.
• [x] I have made sure that all CI jobs on GitHub pass.
• [x] The pull request introduces no new warnings and follows the project's style guidelines.

[codecov[bot]]

Codecov Report

Merging #256 (bb6e4fd) into main (1599eac) will increase coverage by 0.0%. The diff coverage is n/a.

@@          Coverage Diff          @@
##            main    #256   +/-   ##
=====================================
  Coverage   96.0%   96.0%           
=====================================
  Files         34      34           
  Lines       1738    1739    +1     
  Branches     214     214           
=====================================
+ Hits        1670    1671    +1     
  Misses        68      68           

Flag	Coverage Δ
cpp	95.6% <ø> (-0.1%)	:arrow_down:
python	97.6% <ø> (+0.2%)	:arrow_up:

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see 3 files with indirect coverage changes

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