You can try and edit repository and in only Knossos ppa change distribution from eoan to disco.
Thanks for the advice, tomimaki. I was able to figure out how to set that repository to disco rather than eoan. That worked! I was able to install from ngld's repository and tried to install knossos. Unfartunately...
$ sudo apt-get install knossos
[sudo] password for perihelion:
Reading package lists... Done
Building dependency tree
Reading state information... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:
The following packages have unmet dependencies:
knossos : Depends: python3-token-bucket but it is not installable
Depends: python3-raven but it is not installable
E: Unable to correct problems, you have held broken packages.
This sounds vaguely familiar, so I'm going to search a bit through this thread and a few others and see if I'm right and someone else ran into something like this before.
Edit: Yup, Rico and SirCumference had this issue a couple pages ago, though apparently only with the python3-token-bucket dependency, not python3-raven as well. I fixed the token-bucket dependency using the same method they did and started looking around for a repository that had python3-raven. Found a hit on deb https://packages.ubuntu.com/
disco universe. But my Software & Updates started getting attitude when I tried to add that repository and locked up.
So, I think I'm about ready to give up on this install of Ubuntu. I think I steered myself the wrong direction when I aimed for the latest distribution I could find. I should have picked an earlier LTS version. The only reason I installed Ubuntu at all was in the vague hope it would cooperate more with Knossos than Mint 19.3 has been. Otherwise, I much
prefer Mint over Ubuntu. For right now, I'll just have to keep Windows 10 for anything I can't play from wxLauncher in Mint. When I get some free time and some more motivation, I may try to build Knossos 0.14.1RC from source in an isolated copy of Mint or Ubuntu again.