Have I mentioned how much I vastly dislike Debian?!

I think so .... but anyhoo ...

I can't seem to install this PPA repository.

I'm using a Raspberry Pi 4 with "Raspberry Pi OS" (formerly Raspian) -- which is secretly Debian Bookwork (Ubuntu 22.04 LTS) under the covers.

I'm trying to install Python3.12 and (apparently) it's not included in the main OS repositories by default, so after a bunch of Googling, I found I need a 3rd party repo called "Dead Snakes" (because that sounds trustworthy) ... and while the command "apt-add-repository" failed on a python error, I was able to add the line manually in "/etc/apt/sources.list.d/deadsnakes.list" --- but then I get an error about the key missing ...

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--> apt update
Hit:1 http://deb.debian.org/debian bookworm InRelease
Hit:2 http://deb.debian.org/debian-security bookworm-security InRelease
Hit:3 http://deb.debian.org/debian bookworm-updates InRelease
Hit:4 http://archive.raspberrypi.com/debian bookworm InRelease
Get:5 https://ppa.launchpadcontent.net/deadsnakes/ppa/ubuntu jammy InRelease [18.1 kB]
Err:5 https://ppa.launchpadcontent.net/deadsnakes/ppa/ubuntu jammy InRelease
  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY BA6932366A755776
Reading package lists... Done
W: GPG error: https://ppa.launchpadcontent.net/deadsnakes/ppa/ubuntu jammy InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY BA6932366A755776
E: The repository 'https://ppa.launchpadcontent.net/deadsnakes/ppa/ubuntu jammy InRelease' is not signed.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.

===========================

This is the repo URL:  https://ppa.launchpadcontent.net/deadsnakes/ppa/ubuntu/dists/jammy/

and on that page, I see a GPG file, which I tried (probably incorrectly) to install:

curl -fsSL https://ppa.launchpadcontent.net/deadsnakes/ppa/ubuntu/dists/jammy/Release.gpg | gpg --dearmor -o /usr/share/keyrings/deadsnakes.gpg

but then when I do "apt update" I get the error shown above.

This is infinitely easier on RedHat ... just sayin'

Anyhoo -- I appreciate the help on this.  I would prefer to not compile Python from source on a Raspberry Pi, as that takes over an hour to do.