Move python-dateutil from the package's runtime dependencies to its Poetry development group.

Summary

Move python-dateutil from the package's runtime dependencies to its Poetry development group.

Rationale

python-dateutil is currently one of the package's two default dependencies (pyproject.toml:28-32).
In the repository, its only direct import is in the test comparison helper (tests/abstract_sbom_compare.py:13).
The distributed siemens_standard_bom package does not import it.

The project already maintains a Poetry development group, including the corresponding type stubs (pyproject.toml:44-50).

Changes

• Remove python-dateutil from [project].dependencies.
• Add the same version range to [tool.poetry.group.dev.dependencies].

 dependencies = [
-    "python-dateutil (>=2.9.0.post0,<3.0.0)",
     "cyclonedx-python-lib(>=11.11.0,<12.0.0)",
 ]

 [tool.poetry.group.dev.dependencies]
+python-dateutil = ">=2.9.0.post0,<3.0.0"
 types-python-dateutil = "2.9.0.20260716"

Impact

This would keep the test helper available to contributors without adding python-dateutil to every library installation.

If you agree with this change, I would be happy to open a pull request.