"unauthorized_client: Client is unauthorized to retrieve access tokens using this method" Python Gmail API using Service Account

Question

I keep getting the above error and I am not sure how to solve this. I have enabled Domain wide Delegation and also authorized the Client-Id in 'Manage API Access'

My Code looks like this:

"""
Sample program    
"""

import googleapiclient.discovery
import json
import googleapiclient.errors 
from google.oauth2 import service_account 

SERVICE_ACCOUNT_FILE = 'abcd.json' 

credentials = service_account.Credentials.from_service_account_file(
    SERVICE_ACCOUNT_FILE)

scoped_credentials = 
credentials.with_scopes(['https://mail.google.com/'])
delegated_credentials = credentials.with_subject('abcd')

service = googleapiclient.discovery.build('gmail','v1',credentials = delegated_credentials) 

try:
    results = service.users().labels().list(userId='abcd').execute()
except Exception as err:
    print err

Has anyone got this working ? If so please point out why I am getting the error.