Improve flexibility of unit tests with regards to Python versions available in `poetry.core`

[jakob-keller]

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Feature Request

As new Python versions are released, poetry-core needs to be updated to surface them, e.g. in automatically added PyPI trove classifiers (#8375).

This process is tedious due to rigid unit tests in poetry that are run as downstream tests by the poetry-core CI pipeline. Specifically, the tests fail in poetry-core when updating available python versions (python-poetry/poetry-core#631). Adjusting those tests in poetry fails since poetry-core has not been prepared accordingly (#8376).

@radoering proposed the following solution:

I tend to think that we should make these tests more generic so we avoid the dance for each new Python version:
We could change the expectation in the tests to all Python versions in Package.AVAILABLE_PYTHONS that are allowed by the test's Python constraint.

Originally posted by @radoering in #8376 (comment)

I will prepare a PR to address this.

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