At long last, after an unusual wait, Google has made the factory image and OTA file for Android 7.0 “Nougat” available for the Nexus 6P. The build listed by Google is NRD90U, which is different than the previous beta OTA that carried NRD90M. This is likely because you will find the September security patch inside and not August’s.
Have at ’em!
For other Nexus device factory image and OTAs, look here.
UPDATE: Google has now posted images for NRD90T as well.