输出中的错误日志：

NameError at /admin/todo/todomodel/add/
name 'unicode' is not defined
Request Method: POST
Request URL:    http://127.0.0.1:8000/admin/todo/todomodel/add/
Django Version: 2.2
Exception Type: NameError
Exception Value:    
name 'unicode' is not defined

关于系统和异常的更多细节：
异常位置：C:\Users\PEE TEE\Desktop\ToDoApp\lib\site-packages\slugify.py in slugify, line 24
Python可执行文件：C:\Users\PEE TEE\Desktop\ToDoApp\Scripts\python.exe
Python版本：3.7.6
Python路径：
['C:\\Users\\PEE TEE\\Desktop\\ToDoApp\\src', 'C:\\Users\\PEE TEE\\Desktop\\ToDoApp\\Scripts\\python37.zip', 'C:\\Users\\PEE TEE\\Desktop\\ToDoApp\\DLLs', 'C:\\Users\\PEE TEE\\Desktop\\ToDoApp\\lib', 'C:\\Users\\PEE TEE\\Desktop\\ToDoApp\\Scripts', 'c:\\users\\pee tee\\appdata\\local\\programs\\python\\python37\\Lib', 'c:\\users\\pee tee\\appdata\\local\\programs\\python\\python37\\DLLs', 'C:\\Users\\PEE TEE\\Desktop\\ToDoApp', 'C:\\Users\\PEE TEE\\Desktop\\ToDoApp\\lib\\site-packages']
服务器时间：电话：+86-021 - 88888888传真：+86-021 - 88888888
这是我的模型：

from django.db import models
from django.db.models.signals import post_save
from slugify import slugify
# Create your models here.

class ToDoModel(models.Model):
   author = models.ForeignKey('auth.User', on_delete=models.CASCADE) 
   what_todo = models.CharField(max_length=255, default="Event to be done")
   date_todo = models.DateField(auto_now=False,auto_now_add=False) 
   slug = models.SlugField(null=True, blank=True) 

    def save(self, *args, **kwargs):
        if self.what_todo and not self.slug:
            self.slug = slugify(self.what_todo)
        return super(ToDoModel, self).save(*args, **kwargs)

    def __str__(self):
        return str(self.what_todo)