Is /projects/:id/groups implemented #1590

Description of the problem, including code/CLI snippet

Following official documentation below:
https://docs.gitlab.com/ee/api/projects.html#list-a-projects-groups

I would expect to have a way to list the groups currently added on
project, but I do not find the way to do it and I'm not able to see if it's implemented in a sub function (I don't think so, but I'm never too sure :) ).

Expected Behavior

A cli function / args is available either in :

gitlab project-group

or in:

gitlab project group

Actual Behavior

gitlab: error: argument what: invalid choice: 'project-group' 

and

gitlab project: error: argument whaction: invalid choice: 'group'

And nothing in --help seems relevant.

Specifications

• python-gitlab version: python-gitlab-2.10.1
• API version you are using (v3/v4): v4
• Gitlab server version (or gitlab.com): gitlab.com