Description of the problem, including code/CLI snippet

Gitlab now supports GET /projects/:id/issues/:issue_id/related_merge_requests - If someone can outline the steps needed, I'll be happy to contribute a patch

Expected Behavior

Actual Behavior

Specifications

• python-gitlab version: 1.8.0
• API version you are using (v3/v4): v4
• Gitlab server version (or gitlab.com):