Customising Django Auth username length for Django 1.8+

__init__.py

default_app_config = 'myapp.auth.apps.MyAuthAppConfig'

admin.py

from django.contrib import admin
from django.contrib.auth.admin import UserAdmin
from .models import User

admin.site.register(User, UserAdmin)

apps.py

from django.apps import AppConfig


class MyAuthAppConfig(AppConfig):
    name = 'myapp.auth'
    label = 'my_auth'
    verbose_name = 'Authentication and Authorization (*)'

constants.py

USERNAME_MAX_LENGTH = 255

migrations-0001_initial.py

# -*- coding: utf-8 -*-
from __future__ import unicode_literals

from django.db import migrations, models
import django.contrib.auth.models
import django.utils.timezone
import django.core.validators


class Migration(migrations.Migration):

    dependencies = [
        ('auth', '0006_require_contenttypes_0002'),
    ]

    operations = [
        migrations.CreateModel(
            name='User',
            fields=[
                ('id', models.AutoField(primary_key=True, serialize=False, verbose_name='ID', auto_created=True)),
                ('password', models.CharField(verbose_name='password', max_length=128)),
                ('last_login', models.DateTimeField(blank=True, verbose_name='last login', null=True)),
                ('is_superuser', models.BooleanField(help_text='Designates that this user has all permissions without explicitly assigning them.', verbose_name='superuser status', default=False)),
                ('username', models.CharField(error_messages={'unique': 'A user with that username already exists.'}, verbose_name='username', max_length=30, help_text='Required. 30 characters or fewer. Letters, digits and @/./+/-/_ only.', unique=True, validators=[django.core.validators.RegexValidator('^[\\w.@+-]+$', 'Enter a valid username. This value may contain only letters, numbers and @/./+/-/_ characters.', 'invalid')])),
                ('first_name', models.CharField(blank=True, verbose_name='first name', max_length=30)),
                ('last_name', models.CharField(blank=True, verbose_name='last name', max_length=30)),
                ('email', models.EmailField(blank=True, verbose_name='email address', max_length=254)),
                ('is_staff', models.BooleanField(help_text='Designates whether the user can log into this admin site.', verbose_name='staff status', default=False)),
                ('is_active', models.BooleanField(help_text='Designates whether this user should be treated as active. Unselect this instead of deleting accounts.', verbose_name='active', default=True)),
                ('date_joined', models.DateTimeField(verbose_name='date joined', default=django.utils.timezone.now)),
                ('groups', models.ManyToManyField(to='auth.Group', verbose_name='groups', related_query_name='user', related_name='user_set', help_text='The groups this user belongs to. A user will get all permissions granted to each of their groups.', blank=True)),
                ('user_permissions', models.ManyToManyField(to='auth.Permission', verbose_name='user permissions', related_query_name='user', related_name='user_set', help_text='Specific permissions for this user.', blank=True)),
            ],
            options={
                'db_table': 'auth_user',
                'verbose_name': 'user',
                'verbose_name_plural': 'users',
            },
            managers=[
                ('objects', django.contrib.auth.models.UserManager()),
            ],
        ),
    ]

migrations-0002_username_to_255_chars.py

# -*- coding: utf-8 -*-
from __future__ import unicode_literals

from django.db import migrations, models
import django.core.validators


class Migration(migrations.Migration):

    dependencies = [
        ('my_auth', '0001_initial'),
    ]

    operations = [
        migrations.AlterField(
            model_name='user',
            name='username',
            field=models.CharField(error_messages={'unique': 'A user with that username already exists.'}, max_length=255, verbose_name='username', validators=[django.core.validators.RegexValidator('^[\\w.@+-]+$', 'Enter a valid username. This value may contain only letters, numbers and @/./+/-/_ characters.', 'invalid')], help_text='Required. 255 characters or fewer. Letters, digits and @/./+/-/_ only.', unique=True),
        ),
    ]

models.py

from django.core import validators
from django.core.mail import send_mail
from django.contrib.auth.models import UserManager, PermissionsMixin, AbstractBaseUser
from django.db import models
from django.utils.translation import ugettext_lazy as _
from django.utils import timezone
from .constants import USERNAME_MAX_LENGTH


class User(AbstractBaseUser, PermissionsMixin):
    """
    An abstract base class implementing a fully featured User model with
    admin-compliant permissions.

    Username, password and email are required. Other fields are optional.
    """
    class Meta:
        db_table = 'auth_user'
        verbose_name = _('user')
        verbose_name_plural = _('users')

    username = models.CharField(_('username'), max_length=USERNAME_MAX_LENGTH, unique=True,
        help_text=_('Required. {} characters or fewer. Letters, digits and '
                    '@/./+/-/_ only.'.format(USERNAME_MAX_LENGTH)),
        validators=[
            validators.RegexValidator(r'^[\w.@+-]+$',
                                      _('Enter a valid username. '
                                        'This value may contain only letters, numbers '
                                        'and @/./+/-/_ characters.'), 'invalid'),
        ],
        error_messages={
            'unique': _("A user with that username already exists."),
        })
    first_name = models.CharField(_('first name'), max_length=30, blank=True)
    last_name = models.CharField(_('last name'), max_length=30, blank=True)
    email = models.EmailField(_('email address'), blank=True)
    is_staff = models.BooleanField(_('staff status'), default=False,
        help_text=_('Designates whether the user can log into this admin '
                    'site.'))
    is_active = models.BooleanField(_('active'), default=True,
        help_text=_('Designates whether this user should be treated as '
                    'active. Unselect this instead of deleting accounts.'))
    date_joined = models.DateTimeField(_('date joined'), default=timezone.now)

    objects = UserManager()

    USERNAME_FIELD = 'username'
    REQUIRED_FIELDS = ['email']

    def get_full_name(self):
        """
        Returns the first_name plus the last_name, with a space in between.
        """
        full_name = '%s %s' % (self.first_name, self.last_name)
        return full_name.strip()

    def get_short_name(self):
        "Returns the short name for the user."
        return self.first_name

    def email_user(self, subject, message, from_email=None, **kwargs):
        """
        Sends an email to this User.
        """
        send_mail(subject, message, from_email, [self.email], **kwargs)

AUTH_USER_MODEL = 'myapp.User'

in settings.py