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deb https://deb.debian.org/debian/ unstable main contrib non-free deb-src https://deb.debian.org/debian/ unstable main contrib non-free
The simple answer to that is to leave the raspberrypi.org stretch repositories in your sources.list.d directory. You can even leave the raspbian repos in there and priority will be given to the newer Debian versions of anything non-raspberry pi specific.
wow, even better. Debian is simply configurable to any degree of up-to-dateness. Obviously some people don't know that but complain instead:-) There is nothing more to say.