[boronine-bot]

Summary

A complete migration to pyproject.toml, offered as an alternative to the minimal build-system declaration in #75. This PR targets master independently; it does not require #75 to merge.

• Remove setup.py and move package metadata and the single-module declaration to pyproject.toml.
• Keep hsluv.__version__ as the version source via setuptools' dynamic attribute support, rather than importing the module from a setup script.
• Use the SPDX license expression MIT and explicitly include LICENSE.txt. Require setuptools >=77.0.3 for modern license metadata support; the supported Python minimum remains 3.11.
• Remove obsolete setup_requires, test_suite, and setup.cfg's Python 2/3 universal-wheel setting. Wheels are now correctly tagged py3-none-any.
• Replace the outdated README test command with python -m unittest discover -v.
• Update CI to build both distributions with python -m build, install the extracted source distribution, and install and test the wheel from outside the source tree.

Validation

• Built an sdist and a wheel from that sdist with isolated builds and setuptools 84.2.0, without packaging warnings.
• Installed both the sdist and wheel into virtual environments on Python 3.11.17 and 3.14.4. Both tests pass against each installed artifact, with imports verified to resolve to site-packages.
• Verified installed metadata version matches hsluv.__version__ (5.0.4).
• Built both distributions on Python 3.11 with the minimum declared setuptools version, 77.0.3.
• python -m twine check dist/*: both artifacts pass.
• git diff --check: passes.

Compatibility

Runtime code and the Python >=3.11 requirement are unchanged. Building from source requires a PEP 517-capable frontend and setuptools >=77.0.3 (normally supplied by pip's isolated build environment). Direct python setup.py ... commands are no longer available; use python -m build, python -m pip install ., or the documented unittest command instead. Installing published wheels does not require setuptools.