I am not sure if this is common with all of the flavors or just the Corepure64 64. If you install the python3.9.tcz package and pip3 barfs with an error about libexpat.so.1, using tce-ab to install expat2.tcz will make it happy. Perhaps this can get fixed in the dependencies.
As an aside, and I don't claim to be at all good at this, but I looked at the python 3.9 dev and that had tons of X11 stuff, does that being over a GUI or something? Is there a place to get a better description of what a package is than searching for it with tce-ab and reading the first page? I like to keep tiny tiny so I have got so I always look at the dependencies and the tree and the files as well as the size, but the fact there are so many python pieces baffles me. Doesn't pip go and fetch anything you need for you for the most part?