'List of Requirements needed for this distro if `extras` are used'

def requires(self, extras=()):

    dm = self._dep_map
    deps = []
    deps.extend(dm.get(None, ()))
    for ext in extras:
        try:
            deps.extend(dm[safe_extra(ext)])
        except KeyError:
            raise UnknownExtra(('%s    has    no    such    extra    feature    %r' % (self, ext)))
    return deps
