ldap_createsuperuser: Use input() for asking username

This should improve Python3 compatibility

fountain/management/commands/ldap_createsuperuser.py

 # -*- coding: utf-8 -*-
 from __future__ import unicode_literals
 import getpass
-import sys
 from django.core.management.base import BaseCommand
 from django.contrib.auth import get_user_model
 
@@ -23,8 +22,7 @@ class Command(BaseCommand):
         username = options.get('username')
         if not username:
             current_user = getpass.getuser()
-            sys.stdout.write('Username [%s]: ' %current_user)
-            input_user = sys.stdin.readline().strip()
+            input_user = input('Username [{}]: '.format(current_user))
             if not input_user:
                 username = current_user
             else:
@@ -46,3 +44,4 @@ class Command(BaseCommand):
         else:
             obj.save()
             self.stdout.write("Granted %s privileges to user %s." % (privilege, username))
+