I'm using Raspbian Jessie but there are a few packages I want that aren't available (but are in Debian Stretch repos). I want to temporarily use the Stretch repo to install them (and any otherwise-unsatisfied dependencies) but without making anything else come from there in the future.
I understand things might not work; etc.; I'm just trying something out on a throwaway install :)
I tried rigging some files (based on this answer) but I got this.. not sure a) how to fix it and b) whether I'm doing things the right way!
W: GPG error: http://ftp.uk.debian.org stretch InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 8B48AD6246925553 NO_PUBKEY 7638D0442B90D010