[maple3142]

I think the main problem is C modules. CPython only maintains ABI compatibility across minor releases (e.g. 3.10.0 and 3.10.8), so you may need a different binary compiled for CPython 3.9 and 3.10. https://docs.python.org/3/c-api/stable.html

[wiredfool]

It's absolutely an issue with C modules, especially ones that aren't easy to build or if you're running Windows where it's unlikely that there's a compilation environment setup.

Pillow used to _always_ get this when a new Python release came out, because while we were generally following the betas and build away, our quarterly releases weren't sync'd with the Python ones. So there's be a gap, and every. single. time. we'd get support issues with people pip installing on the newest python and not having a compiler or the basic dependencies. (Aside: even if the last line is "Couldn't find required dependency libjpeg" many many people just don't get that it's requiring additional libraries).

So, we just shifted our fall releases to come after the Python releases.

[zimmerfrei]

By default, yes.

However, as the page you link also mentions, a C extension may also opt to use the "Stable ABI", which does not change across major releases (with some caveats).