Fix test_source_with_attributes for python 3.6

tests/test_model_serializer.py

@@ -13,7 +13,6 @@
 
 import django
 import pytest
-from django.contrib.auth.models import User
 from django.core.exceptions import ImproperlyConfigured
 from django.core.serializers.json import DjangoJSONEncoder
 from django.core.validators import (
@@ -737,6 +736,17 @@ class Meta:
         self.assertEqual(repr(TestSerializer()), expected)
 
     def test_source_with_attributes(self):
+        class User(models.Model):
+            username = models.CharField(
+                'username',
+                max_length=150,
+                unique=True,
+                help_text='Required. 150 characters or fewer.',
+            )
+            first_name = models.CharField('first name', max_length=150, blank=True)
+            last_name = models.CharField('last name', max_length=150, blank=True)
+            email = models.EmailField('email address', blank=True)
+
         class UserProfile(models.Model):
             age = models.IntegerField()
             birthdate = models.DateField()
@@ -763,7 +773,7 @@ class Meta:
 
         expected = dedent("""
             UserProfileSerializer():
-                username = CharField(help_text='Required. 150 characters or fewer. Letters, digits and @/./+/-/_ only.', max_length=150, source='user.username', validators=[<django.contrib.auth.validators.UnicodeUsernameValidator object>, <UniqueValidator(queryset=User.objects.all())>])
+                username = CharField(help_text='Required. 150 characters or fewer.', max_length=150, source='user.username', validators=[<UniqueValidator(queryset=User.objects.all())>])
                 email = EmailField(allow_blank=True, label='Email address', max_length=254, required=False, source='user.email')
                 first_name = CharField(allow_blank=True, max_length=150, required=False, source='user.first_name')
                 last_name = CharField(allow_blank=True, max_length=150, required=False, source='user.last_name')