Remove nbformat dependency

Removed nbformat dependency from pixi.toml.
Not present in 4x

Author: davy39

pixi.toml

@@ -15,7 +15,6 @@ xeus-python = "*"
 matplotlib="*"  
 numpy="*" 
 pandas="*"
-nbformat="*"
 ipywidgets="*"
 ipycanvas = "*"
 xeus-sqlite = "*"