Use version numbers specified in Python-Requires to generate pyproject.toml etc.

This MR generalises the packaging.py script to dune modules that have different version numbers than the dependent dune (core) modules.

Version numbers specified in the Python-Requires are now passed through to packagemetadata and, therefore, to pyproject.toml etc.

Compare also: samuel.burbulla/dune-mmesh!14 (closed)