Use get_user_model to determine the user model
Use get_user_model to determine the user model

Instead of directly importing User from django.contrib.auth.models, use
the get_user_model function to determine the configured user model.

This fixes a failure when trying to authenticate in an application that
uses a custom user model.

alandmoore/django_auth_ldap3,sjkingo/django_auth_ldap3