title: 403 error from Google Admin SDK with AppAssertionCredentials
body: <p>I'm trying to list users via Google admin directory API.</p>
  
  <pre><code>import logging
  import os
  
  from google.appengine.api import memcache
  from googleapiclient import discovery
  from oauth2client.contrib.appengine import AppAssertionCredentials
  
  import httplib2
  
  from flask import Flask
  
  
  credentials = AppAssertionCredentials(scope='https://www.googleapis.com/auth/admin.directory.user')
  auth_http = credentials.authorize(httplib2.Http(cache=memcache))
  service = discovery.build('admin', 'directory_v1', http=auth_http)    
  
  @app.route('/list')
  def list():
      results = service.users().list(domain='example.com', maxResults=10, orderBy='email').execute()
      return 'success'
  
  app = Flask(__name__)
  </code></pre>
  
  <p>I'm running this in App Engine and have enabled domain-wide delegation for App Engine default service account, as instructed in <a href="https://developers.google.com/api-client-library/python/auth/service-accounts" rel="nofollow noreferrer">https://developers.google.com/api-client-library/python/auth/service-accounts</a></p>
  
  <p>This is the error I'm getting: <em>HttpError: https://www.googleapis.com/admin/directory/v1/users?orderBy=email&amp;domain=example.com&amp;alt=json&amp;maxResults=10 returned "Not Authorized to access this resource/api"></em></p>