Following the recommendation on this thread, I ran sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 87828F5B 80AD5260 49F9BF71, then sudo apt-get update and got this:
W: GPG error: http://archive.panda3d.org/ubuntu xenial InRelease: The following signatures couldn't
be verified because the public key is not available: NO_PUBKEY BFC01641E375BC9D
W: The repository 'http://archive.panda3d.org/ubuntu xenial InRelease' is not signed.
N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use.
N: See apt-secure(8) manpage for repository creation and user configuration details.
Running sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys E375BC9D solved that (following this thread).
So, next I’m trying sudo apt-get install panda3d1.9 but I get the same ‘unmet dependencies’ error I’ve been getting with dpkg.