Migrate project to use uv (dependency manager)

[lbolla]

This PR migrates the project to use Astral's uv for dependency and project management and setuptools_scm for versioning.

Changes:

• Add pyproject.toml with setuptools_scm dynamic versioning
• Add uv.lock (generated)
• Update Makefile to use uv for venv, installs, linting, and tests
• Add GitHub Actions workflow to build on tag pushes using uv
• Add RELEASE.md and update README.rst with release instructions

Notes for reviewers:

• Publishing to PyPI is commented out in the workflow; add secrets to enable
• EMpy/version.py will be generated by setuptools_scm on build from tags

To test locally:

1. Install uv (installer or pip install uv) if you don't already have it.
2. Create a venv and sync: uv venv && uv sync.
3. Run tests: uv run pytest.

If you'd like I can adjust the CI to automatically publish on tags when secrets are present.