Fix _get_members_set to get role name

[paulnoirel]

Description

This PR fixes the role name of a user group member.
It also makes create and update more robust by making sure that workspace-wide users can't be part of groups.

import labelbox as lb
from labelbox.schema.user_group import UserGroup, UserGroupColor, UserGroupMember

ug_id = "8f5ab120-47ba-11f0-b68a-1370de934330"

client = lb.Client(API_KEY)

user_groups = UserGroup.get_user_groups(client)
example_group = next(
    (group for group in user_groups if group.id == ug_id),
    None
)

if example_group:    
    for member in example_group.members:        
        
        role = member.role
        
        print(f"Role ID: {role.uid}")
        print(f"Role Name: {role.name}")
else:
    print("User group not found")

Before change:

Role ID: cjlvi914y1aa20714372uvzjv
Role Name: None
Role ID: cjlvi919b1aa50714k75euii5
Role Name: None
Role ID: cjlvi914y1aa20714372uvzjv
Role Name: None

After change:

Role ID: cjlvi919b1aa50714k75euii5
Role Name: REVIEWER
Role ID: cjlvi914y1aa20714372uvzjv
Role Name: LABELER
Role ID: cjlvi914y1aa20714372uvzjv
Role Name: LABELER

Fixes # (issue)

Type of change

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• Breaking change (fix or feature that would cause existing functionality to not work as expected)
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