See source file: celery/backends/redis.py

The documentation says:

Note that the ssl_cert_reqs string should be one of required, optional, or none (though, for backwards compatibility, the string may also be one of CERT_REQUIRED, CERT_OPTIONAL, CERT_NONE).

But with 'required':

AttributeError: module 'ssl' has no attribute 'required'

It seems that CERT_REQUIRED, CERT_OPTIONAL and CERT_NONE must be used...