ldap: Add workaround for ldap3 issue #177

This fixes Python3 compatibility
4 jobs for master
Status Job ID Name Coverage
  Test
passed #2125
py27_django18

00:00:07

passed #2126
py27_django19

00:00:07

passed #2127
py34_django18

00:00:09

passed #2128
py34_django19

00:00:09