python: use `embedsignature` for Cython

### What feature or improvement would you like to see?

https://cython.readthedocs.io/en/latest/src/userguide/source_files_and_compilation.html
https://github.com/cython/cython/pull/5415

This might give us better docs/type information in IDEs; we have the .pyi file but not everything necessarily reads that