Support listing unreleased Python versions

[jwodder]

• Add a next_minor_version() (next_release()?) method for getting the next upcoming Python minor version (or major, if that's what's next?)

• Add a next_version_in(major|minor) method that returns the next minor/micro version to be released in the given major/minor version

• Add unreleased_{major,minor,micro}_versions() methods?

• Give pyversion-info list an --unreleased option

• Give the --subversions option of pyversion-info show an unreleased choice