I run a script every Friday to collect and apply updates to a system (still) running Leap 15.3. And every week I have to tell zypper to retain the obsolete jack_mixer after receiving the message:
Is there a reason why that Python module is not in the distribution? The jack_mixer package resides in the packman repository so one might expect ttat it'd the logical home for pygobject, too, right? I note that sources are available for it but I haven't had an opportunity to go down the compile-from-sources path for that module.

Any information on this missing Python module? Is it completely incompatible with newer Python versions?

openSUSE Leap 15.3 is dead, openSUSE Leap 15.4 is short before dead, so install an supported Leap 15.5......

https://en.opensuse.org/Lifetime

Well, that is the obvious answer. That update is on my calendar but for when it's not totally disruptive as this is happening on my primary desktop system. (Frankly, switching to Tumbleweed is looking better every day but I digress.) I've been seeing this error message since before 15.3 was sunsetted. Heck, Python2.7 is still around but something tied to 3.6 is missing? Weird.

Its a bug........

You can report it on the Packman mailing list...........