title: Why I couldn’t use get_user_model() at import time
body: <p>Just as the title, I cannot import usercreationform
  . The exception is model haven’t been loaded. It’s only appears in Django 1.10. There’s no such problem in Django 1.11 . I know it depends on whether get_user_model() can be called at runtime. What should I do to solve it in Django 1.10?</p>