Issue187 pyproject toml - uv build

[Didou09]

Main changes:

Complete refurbishing of packaging:

• setup.cfg, setup.py and requirements.txt removed
• pyproject.toml completed and cleaned-up
  • build back-end: setuptools and setuptool_scm
  • version single-sourcing using an automatically-generated datastock/_version.py file
• Updated .github/workflows/python-testing-matrix.yml to use uv
• Classifiers now live in a separate file

Issues:

Fixes, in devel, issue #187