maint: move static metadata to setup.cfg; update requirements

[mwtoews]

There are several things going on here:

• Move static project metadata to the declarative configuration file setup.cfg, remove setup.py
• gimli.units was missing from requirements.txt / install_requires
• Clean-up non-used items for build-system requirements (Cython and numpy are not used to build the package)

Build tested successfully with python -m build and checked with twine check dist/*.

[mdpiper]

@mwtoews Sorry it's taken so long to get to this, but I have time the next couple of weeks. I've updated the CI workflows. The last thing I'll do is move as much of the static metadata as I can from setup.cfg to pyproject.toml.