Static typing

traitlets' types are used to perform run-time checks. Would adding type annotation on them make sense?
It seems that we have some information that we could use to potentially find bugs ahead of time. For instance, should a `Bool` be statically typed as a `bool`? I don't even know if that makes sense :smile:
One problem I'm seeing (for instance in https://github.com/jupyter/nbclient) is that adding type annotations on traits doesn't seem to have any effect.

Start by investigating traitlets' runtime type checks and the annotation behavior observed in nbclient. Clarify whether traits such as Bool can expose static types such as bool, and define tests or examples that demonstrate the intended behavior.

Scritto dal modello di indicizzazione a partire dal testo della issue.