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.