Add support for subgroups listing

Closes #390

docs/gl_objects/groups.rst

@@ -72,6 +72,25 @@ Remove a group:
    :start-after: # delete
    :end-before: # end delete
 
+Subgroups
+=========
+
+Reference
+---------
+
+* v4 API:
+
+  + :class:`gitlab.v4.objects.GroupSubgroup`
+  + :class:`gitlab.v4.objects.GroupSubgroupManager`
+  + :attr:`gitlab.v4.objects.Group.subgroups`
+
+Examples
+--------
+
+List the subgroups for a group::
+
+    subgroups = group.subgroups.list()
+
 Group members
 =============
 

gitlab/v4/objects.py

@@ -554,6 +554,18 @@ class GroupProjectManager(GetFromListMixin, RESTManager):
                      'ci_enabled_first')
 
 
+class GroupSubgroup(RESTObject):
+    pass
+
+
+class GroupSubgroupManager(GetFromListMixin, RESTManager):
+    _path = '/groups/%(group_id)s/subgroups'
+    _obj_cls = GroupSubgroup
+    _from_parent_attrs = {'group_id': 'id'}
+    _list_filters = ('skip_groups', 'all_available', 'search', 'order_by',
+                     'sort', 'statistics', 'owned')
+
+
 class GroupVariable(SaveMixin, ObjectDeleteMixin, RESTObject):
     _id_attr = 'key'
 
@@ -570,11 +582,12 @@ class Group(SaveMixin, ObjectDeleteMixin, RESTObject):
     _short_print_attr = 'name'
     _managers = (
         ('accessrequests', 'GroupAccessRequestManager'),
+        ('issues', 'GroupIssueManager'),
         ('members', 'GroupMemberManager'),
         ('milestones', 'GroupMilestoneManager'),
         ('notificationsettings', 'GroupNotificationSettingsManager'),
         ('projects', 'GroupProjectManager'),
-        ('issues', 'GroupIssueManager'),
+        ('subgroups', 'GroupSubgroupManager'),
         ('variables', 'GroupVariableManager'),
     )
 

tools/python_test_v4.py

@@ -206,6 +206,7 @@
 assert(len(gl.groups.list()) == 3)
 assert(len(gl.groups.list(search='oup1')) == 1)
 assert(group3.parent_id == p_id)
+assert(group2.subgroups.list()[0].id == group3.id)
 
 group1.members.create({'access_level': gitlab.Group.OWNER_ACCESS,
                        'user_id': user1.id})