PoetryFunTest: Investigate why `isort` is not listed as direct dependency

[fviernau]

          BTW, do you have any idea why `isort` is not also listed as a direct dependency here, like it is in `pyproject.toml` under `[tool.poetry.dev-dependencies]`?

Originally posted by @sschuberth in https://github.com/oss-review-toolkit/ort/pull/7632#discussion_r1350096958

[fviernau]

A quick investigation showed that isort is in the requirements generated via poetry export, but is not contained with the first level entries of the resolved_dependencies_graph properties within the JSON result provided by python-inspector.

Adding the "bug" label as this might be a bug (not 100% certain).

[sschuberth]

Maybe my observation over here are related.