v2.3.0rc2

Switch from nox to uv for development and CI (#335)

Replace the nox-based workflow with uv:
- Remove noxfile.py; use `uv run pytest` and `uvx pre-commit` instead
- Add a `dev` dependency group for the test tooling
- Update the GitHub Actions workflow to use astral-sh/setup-uv, `uv run`
  for tests, and `uv build` for the sdist/wheel
- Update CONTRIBUTING.md and docs/install.rst for the new commands
- Ignore `.venv` and `uv.lock` in .gitignore

Also regenerate the reference test images to match the versions of
matplotlib used on CI.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>