Prepare for 4.0.0 release

[jcarbaugh]

Summary

This PR modernizes the project's packaging, tooling, and CI configuration in preparation for the 4.0.0 release — the first new release in several years.

The project was already in good shape (uv, ruff, GitHub Actions, OIDC publishing to PyPI) so this is a targeted cleanup pass rather than a full overhaul.

Changes

Packaging (pyproject.toml)

• Added missing build-backend = "setuptools.build_meta" to [build-system] (required by PEP 517)
• Moved from dynamic = ["version"] (setuptools attribute lookup) to a static version = "4.0.0.dev0" — cleaner and compatible with uv version for release automation

Version module (us/version.py)

• Replaced hardcoded __version__ string with importlib.metadata.version("us") so the reported version always matches the installed package
• Added a try/except fallback to "unknown" for environments where the package metadata isn't available

Release workflow (.github/workflows/publish.yml)

• Replaced the fragile sed regex that patched version.py at release time with uv version "${GITHUB_REF#refs/tags/v}", which sets the version directly in pyproject.toml
• To release: push a tag v4.0.0 → CI runs tests → uv version sets the version → uv build → publishes to PyPI via Trusted Publishing (no API token needed)

Documentation

• README.md: fixed uv install us (not a valid command) → uv add us; replaced uv run black --check us with the correct ruff commands
• CONTRIBUTING.md: updated formatting tool reference from black to ruff

[jcarbaugh]

Changes to focus on in review:

• us/version.py — switched from a hardcoded string to importlib.metadata.version("us") with a try/except fallback. Worth a second look: the fallback returns "unknown" rather than raising, which is intentional for environments where the package isn't installed (e.g. bare sys.path import without uv sync).

• publish.yml — the sed version-patching step is replaced by uv version "${GITHUB_REF#refs/tags/v}". This modifies pyproject.toml in-place during the CI run (not committed back). uv build reads it fresh, so the published wheel picks up the correct version.

• pyproject.toml — version is now static (4.0.0.dev0); dynamic = ["version"] and [tool.setuptools.dynamic] are removed. The build-backend was also missing and is now added.