ImportError: cannot import name '...' from partially initialized module '...' (most likely due to a circular import)

I'm upgrading an application from Django 1.11.25 (Python 2.6) to Django 3.1.3 (Python 3.8.5) and, when I run manage.py makemigrations, I receive this message:

File "/home/eduardo/projdevs/upgrade-intra/corporate/models/section.py", line 9, in <module>
    from authentication.models import get_sentinel**

ImportError: cannot import name 'get_sentinel' from partially initialized module 'authentication.models' (most likely due to a circular import) (/home/eduardo/projdevs/upgrade-intra/authentication/models.py)**

My models are:

authentication / models.py

from django.conf import settings
from django.contrib.auth.models import AbstractUser, UserManager
from django.db import models
from django.db.models.signals import post_save
from django.utils import timezone

from corporate.constants import GROUP_SUPPORT
from corporate.models import Phone, Room, Section
from library.exceptions import ErrorMessage
from library.model import update_through_dict
from .constants import INTERNAL_USER, EXTERNAL_USER, SENTINEL_USERNAME, SPECIAL_USER, USER_TYPES_DICT


class UserProfile(models.Model):
    user = models.OneToOneField(
        'User',
        on_delete=models.CASCADE,
        unique=True,
        db_index=True
    )
    ...
    phone = models.ForeignKey('corporate.Phone', on_delete=models.SET_NULL, ...)
    room = models.ForeignKey('corporate.Room', on_delete=models.SET_NULL, ...)
    section = models.ForeignKey('corporate.Section', on_delete=models.SET_NULL, ...)
    objects = models.Manager()
    ...

class CustomUserManager(UserManager):

    def __init__(self, type=None):
        super(CustomUserManager, self).__init__()
        self.type = type

    def get_queryset(self):
        qs = super(CustomUserManager, self).get_queryset()
        if self.type:
            qs = qs.filter(type=self.type).order_by('first_name', 'last_name')
        return qs

    def get_this_types(self, types):
        qs = super(CustomUserManager, self).get_queryset()
        qs = qs.filter(type__in=types).order_by('first_name', 'last_name')
        return qs

    def get_all_excluding(self, types):
        qs = super(CustomUserManager, self).get_queryset()
        qs = qs.filter(~models.Q(type__in=types)).order_by('first_name', 'last_name')
        return qs

class User(AbstractUser):
    type = models.PositiveIntegerField('...', default=SPECIAL_USER)
    username = models.CharField('...', max_length=256, unique=True)
    first_name = models.CharField('...', max_length=40, blank=True)
    last_name = models.CharField('...', max_length=80, blank=True)
    date_joined = models.DateTimeField('...', default=timezone.now)
    previous_login = models.DateTimeField('...', default=timezone.now)

    objects = CustomUserManager()
    ...
    def get_profile(self):
        if self.type == INTERNAL_USER:
            ...
        return None

    def get_or_create_profile(self):
        profile = self.get_profile()
        if not profile and self.type == INTERNAL_USER:
            ...
        return profile

    def update(self, changes):
        ...

class ExternalUserProxy(User):
    objects = CustomUserManager(type=EXTERNAL_USER)

    class Meta:
        proxy = True
        verbose_name = '...'
        verbose_name_plural = '...'

class InternalUserProxy(User):
    objects = CustomUserManager(type=INTERNAL_USER)

    class Meta:
        proxy = True
        verbose_name = '...'
        verbose_name_plural = '...'

def create_profile(sender, instance, created, **kwargs):
    if created and instance.type == INTERNAL_USER:
        try:
            profile = UserProfile()
            profile.user = instance
            profile.save()
        except:
            pass

post_save.connect(create_profile, sender=User)

def get_sentinel():
    try:
        sentinel = User.objects.get(username__exact=SENTINEL_USERNAME)
    except User.DoesNotExist:
        settings.LOGGER.error("...")
        from django.contrib.auth.models import Group
        sentinel = User()
        sentinel.username = SENTINEL_USERNAME
        sentinel.first_name = "..."
        sentinel.last_name = "..."
        sentinel.set_unusable_password()
        sentinel.save()
        technical = Group.objects.get(name=GROUP_SUPPORT)
        sentinel = User.objects.get(username__exact=SENTINEL_USERNAME)
        sentinel.groups.add(technical)
        sentinel.save()
    return sentinel

corporate / models / __init__.py

...
from .section import Section
...

corporate / models / section.py

from django.conf import settings
from authentication.models import get_sentinel
from .room import Room

class Section(models.Model):
    ...
    boss = models.ForeignKey(settings.AUTH_USER_MODEL, on_delete=models.SET(get_sentinel), ...)
    surrogate = models.ForeignKey(settings.AUTH_USER_MODEL, on_delete=models.SET(get_sentinel), ...)
    room = models.ForeignKey(Room, on_delete=models.SET_NULL, ...)
    is_subordinate_to = models.ForeignKey('self', on_delete=models.SET_NULL, ...)
    ...

What am I doing wrong?

You have a circular import.

authentication/models imports corporate/models, which imports corporate/models/section, which imports authentication/models.

You can't do that. Rewrite and/or rearrange your modules so that circular imports aren't needed.

One strategy to do this is to organize your modules into a hierarchy, and make sure that a module only imports other modules that are lower in the hierarchy. (This hierarchy can be an actual directory structure, but it doesn't have to be; it can just be a mental note in the programmer's mind.)