Update packaging to use uv and uv-build

[mdickinson]

This PR updates packaging and development tasks to use uv.

• Drop support for Python 3.8 and Python 3.9.
• Add a DEVELOP.md file describing how to carry out common development tasks.
• Use uv_build instead of setuptools as the build backend.
• Drop use of setuptools_scm for versioning. We'll version manually instead.
• Include Python 3.14 free-threaded build in the test matrix.
• Move development requirements into pyproject.toml, under dependency-groups.
• Commit the uv.lock file for reproducible builds.