build: use poetry_core to build instead of poetry

[dtrodrigues]

Description

Changes the build backend from poetry to poetry_core. poetry_core is lighter weight than poetry and all that's required for the build vs project management aspect of poetry.

Checklist

• Add test cases to all the changes you introduce
• Run ./script/format and ./script/test locally to ensure this change passes linter check and test
• Test the changes on the local machine manually
• Update the documentation for the changes

Expected behavior

Steps to Test This Pull Request

Additional context

https://python-poetry.org/docs/pyproject/#poetry-and-pep-517

If your pyproject.toml file still references poetry directly as a build backend, you should update it to reference poetry-core instead.

[Lee-W]

I'm good with it. Seem to be the recommended way https://python-poetry.org/docs/pyproject/#poetry-and-pep-517

@woile what do you think?

[codecov]

Codecov Report

Merging #460 (1216a9e) into master (48f9b7e) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #460   +/-   ##
=======================================
  Coverage   97.96%   97.96%           
=======================================
  Files          39       39           
  Lines        1424     1424           
=======================================
  Hits         1395     1395           
  Misses         29       29           

Flag	Coverage Δ
unittests	97.96% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
commitizen/__version__.py	100.00% <100.00%> (ø)

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