💚 Fix CI installs and tests

[tiangolo]

💚 Fix CI installs and tests.

Poetry 1.2.0ax currently has an issue that was solved here: python-poetry/poetry-core#188, but is not released in the alpha branches.

This project uses Poetry 1.2.0ax to be able to use the Poetry Version Plugin https://github.com/tiangolo/poetry-version-plugin

Until there's a new release with that fix, this change installs poetry-core from Git, pinned to the version that fixes the problem.

This PR also fixes several dangling issues detected by the first CI runs, missing dev dependencies, type annotations, etc.

[codecov]

Codecov Report

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The diff coverage is n/a.

@@           Coverage Diff           @@
##             main       #2   +/-   ##
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  Coverage        ?   97.24%           
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  Files           ?      175           
  Lines           ?     5842           
  Branches        ?        0           
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  Hits            ?     5681           
  Misses          ?      161           
  Partials        ?        0           

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[github-actions]

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