management: ldap_createsuperuser: Use `six.moves` for `input()` function

Commit b7b75d69 introduced a regression
for Python2 projects where the username input prompt does not work any
more. This can be worked around by using the `input()` function from
`six.moves`.
6 jobs for master in 38 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
passed #33601
py27-django110

00:00:20

89.0%
passed #33602
py27-django111

00:00:23

89.0%
passed #33600
py27-django18

00:00:20

89.0%
passed #33604
py36-django110

00:00:21

90.0%
passed #33605
py36-django111

00:00:16

90.0%
passed #33603
py36-django18

00:00:22

90.0%