doesn't declare an explicit app_label and isn't in an application in INSTALLED_APPS.

在settings.py中增加

INSTALLED_APPS = [
    ...
    'django.contrib.sites',
]

问题就解决了。什么原因。——不知道。。

 

具体请看：

https://stackoverflow.com/questions/35388637/runtimeerror-model-class-django-contrib-sites-models-site-doesnt-declare-an-ex