Update build-backend to use poetry-core

[mweinelt]

Checklist

• [x] Read the Contributing Guidelines and Code of Conduct
• [ ] An issue has been opened to link to this PR
• [x] Each commit encompasses the smallest logical changeset
• [x] Each commit message is written in a well-formed, consistent manner
• [ ] Issue number(s) are in the commit message as prescribed
• [ ] Link appropriate issues to the PR from the menu on the right

Additional context/information

The build backend of poetry was split into a separate package called poetry-core. This updates the pyproject.toml to reflect that change.

For more info, see https://github.com/python-poetry/poetry-core#poetry-core

[codecov[bot]]

Codecov Report

Merging #41 (dfe91a6) into main (4931aa5) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##             main      #41   +/-   ##
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  Coverage   91.57%   91.57%           
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  Files          29       29           
  Lines        3287     3287           
  Branches      370      370           
=======================================
  Hits         3010     3010           
  Misses        225      225           
  Partials       52       52           

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