Support PEP-735 dependency-groups

[lordmauve]

PEP-735 adds support for groups of package dependencies at a project level. These packages are intended to capture dev tools needed for the projects.

Unlike extras, dependency-groups are not exposed in the built package. Indeed dependency-groups do not imply that the package itself is to be installed.

The intention and flexibility of dependency groups makes them a more appropriate system for a dev tool like thx than extras. We should add support for dependency groups alongside extras.