What major packaging system allows for multiple parallel installations of the same library? Not just in Python, in anything?
Gentoo's portage has SLOTs. AFAIK this is exactly it. I have many standard libs in multiple versions on my system.
With Portage different versions of a single package can coexist on a system. While other distributions tend to name their package to those versions (like freetype and freetype2) Portage uses a technology called SLOTs. An ebuild declares a certain SLOT for its version. Ebuilds with different SLOTs can coexist on the same system. For instance, the freetype package has ebuilds with SLOT="1" and SLOT="2" .