Earlier quoted context omitted.
Debian also does not have anything above python 3.4 on the stable packages branch 3.6 has been dev and test since 2016
Why in the world is Debian like this? Their buggy latexmk is also from 2015. Somehow I feel like every system that I want to keep updated stays on an old version, and every system that I want to keep pinned to some version just force-feeds me its updates...
There is backports, and I've thought about maintaining a newer Python interpreter in backports (when I was working for a company using Jessie and trying to use a Debian stack for everything and sort of wanting async syntax). You could introduce python3.6 or python3.7 as a new package in backports. But it would be extremely hard to update the Python modules in Debian to the newer version, too, because there's just a "python3-foo" package for each module, so you'd either have to recompile all binary modules and patch them to work on the newer Python as necessary (and patch anything using async or await as regular identifiers....), or you'd have to introduce an interpreter that intentionally wasn't compatible with modules installed as Debian packages. (Or you could introduce an interpreter that tried its best and failed randomly, which seems worse than either option.) I'm personally sympathetic to wanting to get /usr/bin/python3.6 from my OS even if the only useful thing to do is put a virtualenv on top, but I think it makes sense to just install Python yourself into /usr/local at that point.
Re latexmk, it looks like the volunteer maintainer hasn't updated it since 2015, and there's a culture of not stepping on people's toes and waiting a while before assuming they're inactive or no longer care. But it's been updated in the development release. The current changelog https://metadata.ftp-master.debian.org/changelogs/main/l/lat... has a bunch of "NMU" entries, non-maintainer uploads.