Features
- Refactor JSON metadata code. Now we use `hatchling` to generate
conventionall RFC 822-formatted distribution metadata, then convert
that to JSON, explicitly following the steps outline in [PEP
566](https://peps.python.org/pep-0566/#json-compatible-metadata).
Among other things, this allows configuration of the
*Metadata-Version* by setting
`tool.hatch.build.targets.zipped-directory.core-metadata-version`.
Tests
- We now have 100% test coverage.