title: ldapdb: "'NoneType' object is not callable" when adding ldap database to django
body: <p>Running django 1.11 and trying to connect to LDAP server by using django module ldapdb (django-ldapdb==0.8.0):</p>
  
  <pre><code>DATABASES = {
      'ldap': {
          'ENGINE': 'ldapdb.backends.ldap',
          'NAME': 'ldap://192.168.1.39/',
      },
      'default': {
          'ENGINE': 'django.db.backends.postgresql_psycopg2', # Add 'postgresql_psycopg2', 'mysql', 'sqlite3' or 'oracle'.
          'NAME': 'prot',                      # Or path to database file if using sqlite3.
          'USER': 'prot',                      # Not used with sqlite3.
          'PASSWORD': 'geheim',                  # Not used with sqlite3.
          'HOST': 'localhost',                      # Set to empty string for localhost. Not used with sqlite3.
          'PORT': '5432',                      # Set to empty string for default. Not used with sqlite3.
      },
  }
  
  DATABASE_ROUTERS = ['ldapdb.router.Router']
  </code></pre>
  
  <p>However I get an error in the ldapdb code.</p>
  
  <pre><code>Unhandled exception in thread started by &lt;function check_errors.&lt;locals&gt;.wrapper at 0x7f4ba61ff840&gt;
  Traceback (most recent call last):
    File "/home/nfs/ruben/Projects/prot-zboss/venv/lib/python3.4/site-packages/django/utils/autoreload.py", line 227, in wrapper
      fn(*args, **kwargs)
    File "/home/nfs/ruben/Projects/prot-zboss/venv/lib/python3.4/site-packages/django/core/management/commands/runserver.py", line 125, in inner_run
      self.check(display_num_errors=True)
    File "/home/nfs/ruben/Projects/prot-zboss/venv/lib/python3.4/site-packages/django/core/management/base.py", line 359, in check
      include_deployment_checks=include_deployment_checks,
    File "/home/nfs/ruben/Projects/prot-zboss/venv/lib/python3.4/site-packages/django/core/management/base.py", line 346, in _run_checks
      return checks.run_checks(**kwargs)
    File "/home/nfs/ruben/Projects/prot-zboss/venv/lib/python3.4/site-packages/django/core/checks/registry.py", line 81, in run_checks
      new_errors = check(app_configs=app_configs)
    File "/home/nfs/ruben/Projects/prot-zboss/venv/lib/python3.4/site-packages/django/core/checks/model_checks.py", line 30, in check_all_models
      errors.extend(model.check(**kwargs))
    File "/home/nfs/ruben/Projects/prot-zboss/venv/lib/python3.4/site-packages/django/db/models/base.py", line 1284, in check
      errors.extend(cls._check_long_column_names())
    File "/home/nfs/ruben/Projects/prot-zboss/venv/lib/python3.4/site-packages/django/db/models/base.py", line 1699, in _check_long_column_names
      connection = connections[db]
    File "/home/nfs/ruben/Projects/prot-zboss/venv/lib/python3.4/site-packages/django/db/utils.py", line 212, in __getitem__
      conn = backend.DatabaseWrapper(db, alias)
    File "/home/nfs/ruben/Projects/prot-zboss/venv/lib/python3.4/site-packages/ldapdb/backends/ldap/base.py", line 187, in __init__
      super(DatabaseWrapper, self).__init__(*args, **kwargs)
    File "/home/nfs/ruben/Projects/prot-zboss/venv/lib/python3.4/site-packages/django/db/backends/base/base.py", line 96, in __init__
      self.client = self.client_class(self)
  TypeError: 'NoneType' object is not callable
  </code></pre>
  
  <p>The ldapdb 0.8 is the last version and supports django >=1.10.</p>