Extract nanobind multiple inheritance support to separate project?

[virtuald]

I embarked on an adventure to port multiple inheritance support from pybind11 to nanobind via gpt-5.3-codex. I got pretty far, and it occurred to me that probably one could implement this without actually modifying nanobind at all (or at least, in a very limited way).

As I was contemplating nb_type_get, I started searching for other projects and found your polymorphic type caster implementation and the related MI helpers.... so I guess I was right, one can do that. :)

MI is a really common need, so it seems to me that extracting the code into a separate pypi project would be a pretty reasonable thing to do. Are you interested in doing this? If not, I'll probably swipe some of it and do it myself.