GitLab 10.3 introduces functionality to get a groups subgroups (https://docs.gitlab.com/ce/api/groups.html#list-a-groups-s-subgroups). I would like to have this supported in the Python API.

This functionality is quite essential, because in GitLab 10.3, it now only lists the 'root' groups with Gitlab.groups.list(all_available=True), so using the python-gitlab api it is no longer possible to get access to subgroups if you don't know their exact names.