Update build-backend to use poetry-core

[mweinelt]

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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]

Codecov Report

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

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

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