Fix test for Django 3.0

tests/test_model_serializer.py

@@ -761,11 +761,15 @@ class TestRelationalFieldMappings(TestCase):
                     }
                 }
 
-        expected = dedent("""
+        # In Django 3.0, the maximum length of first_name is 30, whereas it is 150
+        # in later versions, so we can't hard-code the value in the expected variable.
+        max_length = User.first_name.field.max_length
+
+        expected = dedent(f"""
             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())>])
                 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')
+                first_name = CharField(allow_blank=True, max_length={max_length}, required=False, source='user.first_name')
                 last_name = CharField(allow_blank=True, max_length=150, required=False, source='user.last_name')
                 age = IntegerField()
                 birthdate = DateField()