v0.2.2: declare the MIT license in the package metadata

- pyproject.toml records the license via the PEP 639 fields, so the
  distributions carry License-Expression: MIT and ship LICENSE. Releases
  up to 0.2.1 published with no license set.
- Build requirement bumped to setuptools >= 77, the first version that
  supports those fields.
- No functional change to the library.