nbdev-bump-version unable to specify part

FastStripe uses 4 parts in its versioning scheme. In nbdev2, I could use `nbdev_bump_version --part 3` in order to increment the last part. However, now with nbdev3 it no longer does that. Instead it bumps removes the last part of my versioning scheme

<img width="957" height="129" alt="Image" src="https://github.com/user-attachments/assets/57d904b8-0abf-4bd3-b47e-acbf6363fee1" />