Historically this was needed as a RUN_DEPENDS due to consumers using the included pkg_resources, which has been deprecated for some time now in favour of the importlib suite. (pkg_resources consumers are strongly advised to migrate to importlib.)
Additionally, due to environment pollution, this cannot be specified as a RUN_DEPENDS anywhere; consumers of consumers that need this functionality (setuptools_scm comes to mind) are to specify this in BUILD_DEPENDS only. Otherwise certain dependency trees are wrong, which can prevent tests from being available amongst other things.