Add `PackageMetadata.authors` and `PackageMetadata.maintainers` to the
`importlib.metadata` module. These unify and provide minimal parsing for
the respective core metadata fields ("Author", "Author-email"), and
("Maintainer", "Maintainer-email").

A hypothesis strategy for generating structured core metadata and
equivalent unstructured text. Ensures that parsing the text using
PackageMetadata results in the same structure - a roundtrip test.

* Document `PackageMetadata.authors` and `PackageMetadata.maintainers`
  in the usage guide.
* Export `Ident` so it can be documented but also used to build custom
  parsing strategies.

Run the `vermin` tool to identify and fix code incompatible with Python
>= 3.8.

Properties `PackageMetadata.authors` and `PackageMetadata.maintainers`
are now lists with repeated elements removed, rather than sets.