Move project metadata to pyproject.toml, use dependency groups

[mwtoews]

This moves the project metadata to pyproject.toml following PEP 621, as supported by setuptools. This is intended as a minimal conversion, but there are a few other changes:

• Use improved license metadata (PEP 639), which requires setuptools>=77
• Use dependency groups (PEP 735) to provide dependencies for development
  • The dev extra is removed -- extras are published with the metadata to PyPI and should normally be user-facing and not for developers.
  • Developers need a recent version of pip (v25.1) to do: pip install --group dev (can be combined with -e . for an "editable" project install)
  • Developers that use uv can simply type uv sync (since the dev group is included by default)
  • Individual development groups lint, tests and docs are also created, to help organise these a bit, or if they need to be installed for a single purpose, e.g. building docs.
• Remove setup.cfg and setup.py files, as they are no longer needed
• Add *.lock to .gitignore to ignore lock files, e.g. created by uv, pixi and others.

This pr does not attempt to reconfigure how the "requirements*txt" files work, as it' not sure what their purpose is.