Add and/or check type-hints for the following files
    gitlab.v4.objects.access_requests
    gitlab.v4.objects.applications
    gitlab.v4.objects.broadcast_messages
    gitlab.v4.objects.deployments
    gitlab.v4.objects.keys
    gitlab.v4.objects.merge_trains
    gitlab.v4.objects.namespaces
    gitlab.v4.objects.pages
    gitlab.v4.objects.personal_access_tokens
    gitlab.v4.objects.project_access_tokens
    gitlab.v4.objects.tags
    gitlab.v4.objects.templates
    gitlab.v4.objects.triggers

Add a 'get' method with the correct type for Managers derived from
GetMixin.

 * Add type-hints to gitlab/v4/objects/groups.py
 * Have share() function update object attributes.
 * Add 'get()' method so that type-checkers will understand that
   getting a group is of type Group.

 * Add type-hints to gitlab/v4/objects/merge_requests.py
 * Add return value to cancel_merge_when_pipeline_succeeds() function
   as GitLab docs show it returns a value.
 * Add return value to approve() function as GitLab docs show it
   returns a value.
 * Add 'get()' method so that type-checkers will understand that
   getting a project merge request is of type ProjectMergeRequest.

Add links to the GitLab API docs for merge_requests.py as it contains
code which spans two different API documentation pages.

feat(api): add merge trains

Adds promotion to Project Milestones

Signed-off-by: Raimund Hook <raimund.hook@exfo.com>

Add support for merge trains

feat(api): add merge request approval state