Registration form is incompatible with custom user models

[AntonGrigorievich]

as you can see here
¶ If you reference User directly (for example, by referring to it in a foreign key), your code will not work in projects where the AUTH_USER_MODEL setting has been changed to a different user model.

In your RegistrationForm you do just that, which causes an error Manager isn't available; 'auth.User' has been swapped for 'users.User'

As I see it only happens in one form in forms.py, so fixing this problem and making your users' lives will be very easy.

from django.contrib.auth import get_user_model
# model = User
model = get_user_model()