Update to use new license expressions syntax in pyproject.toml

[nightlark]

License classifiers are deprecated and slated for removal in early 2026. We should use the newer license expression syntax (https://packaging.python.org/en/latest/guides/writing-pyproject-toml/#license), which will involve using a newer version of setuptools (>=77.0.3) or switching to hatchling (>=1.27.0). If we switch to hatchling, we'd also want to switch to hatch-vcs. The main benefit of hatchling is that it can create wheels faster than setuptools.