Django从1.10升级到1.11的DeprecationWarning

我正在尝试将我的项目从Django 1.10升级到1.11：\n

>>> django.VERSION
(1, 10, 0, 'final', 1)

\n当我运行python -Wall manage.py test时，我收到以下错误信息：\n

C:\Users\Environments\lib\site-packages\django\contrib\auth\base_user.py:52: DeprecationWarning: __class__ not set defining 'AbstractBaseUser' as . Was __classcell__ propagated to type.__new__? class AbstractBaseUser(models.Model):

\n我已经查找了一些解决方案，但没有找到解决这个问题的方法。鉴于我正在扩展用户类，这应该不仅仅是针对我的项目。所以，有人找到了解决办法吗？\n

class User(AbstractBaseUser, PermissionsMixin):
    """
    自定义用户类
    """
    USER_TYPE = (
        ('owner', 'Owner'),
        ('developer', 'Developer'),
        ('general contractor', 'General Contractor'),
        ('general contractor\'s employee', 'General Contractor\'s Employee'),
        ('consultant', 'Consultant'),
        ('subcontractor', 'Subcontractor'),
        ('home owners', 'Home Owners'),
        ('construction financier', 'Construction Financier'),
        ('lawyer', 'Lawyer'),
        ('accountant', 'Accountant'),
        )
    email = models.EmailField(verbose_name = 'email address',unique = True, db_index = True)
    # email是用于标识目的的唯一字段
    joined = models.DateTimeField(auto_now_add = True)
    is_active = models.BooleanField(default = True)
    is_admin = models.BooleanField(default = False)
    is_superuser = models.BooleanField(default = False)
    #is_staff = models.BooleanField(default = False)
    user_type = models.CharField(max_length = 30, choices = USER_TYPE)
    group = models.ManyToManyField(Group, related_name = 'users')
    permission = models.ManyToManyField(Permission, related_name = 'users')
    objects = CustomUserManager()
    # Added:
    #company = models.ForeignKey(Company, related_name = 'users')
    USERNAME_FIELD = 'email'  # 唯一标识符（必填）该字段在其定义中必须设置unique=True（见上文）
    def get_full_name(self):
        return self.email
    def get_short_name(self):
        return self.email
    def has_perm(self, perm, obj=None):
        ''' 用户是否具有特定权限 '''
        return True   # 这可能需要根据我们想要查找权限的对象进行修改
    def has_module_perms(self, app_label):
        ''' 用户是否有权限查看应用程序'app_label'？默认答案是是。
        这可能会在以后进行修改。 '''
        return True
    @property
    def is_staff(self):
        ''' 用户是否是员工的成员？ '''
        return self.is_admin
    def __unicode__(self):
        return '{user_email}, {user_title} joined on {joined_date}'.format(user_email = self.email,
                                                                           user_title = self.user_type,
                                                                           joined_date = self.joined)
    def __str__(self):
        return '{user_email}, {user_title} joined on {joined_date}'.format(user_email = self.email,
                                                                           user_title = self.user_type,
                                                                           joined_date = self.joined)