migrate to pyproject.toml

[branchv]

👋 Just helping modernize projects by migrating to pyproject.toml (similar to glom). For the most part, things are identical except for updating the license metadata to take advantage of PEP 639 and parsing version_info from __version__ to help flit statically detect it.

To sanity check the new build:

$ uv build -o ./dist/before
$ git switch pyproject && uv build -o ./dist/after
$ uvx diffoscope --exclude-directory-metadata=recursive ./dist/before/*.tar.gz ./dist/after/*.tar.gz