Thanks Frank Bijlsma for the initial patch.

It's not clear when it started to work.

Thanks to Christian von Roques for the report.

This makes it equivalent to: `from django.contrib.auth.models import User`.

Thanks Aymeric Augustin for the initial patch and Tim Graham for the
review.

Identity comparison is quicker than isinstance(), and it avoids coupling
between Model and AutoField.

Fixed #27295 -- Added a system check to prohibit model names that start or end with an underscore or contain double underscores.