django.core.exceptions.ImproperlyConfigured: Requested setting AUTHENTICATION_BACKENDS, but settings are not configured

django.core.exceptions.ImproperlyConfigured: Requested setting AUTHENTICATION_BACKENDS, but settings are not configured. You mu
st either define the environment variable DJANGO_SETTINGS_MODULE or call settings.configure() before accessing settings.

 需要使用到Django 的数据库，Django数据库需要使用的话必须先到settings 里设置

把manage.py 里面的

os.environ.setdefault("DJANGO_SETTINGS_MODULE", "projectname.settings")

这一句放在自定义的manage.py里面