Remove dep on traitlet

Hi!
as we discovered, the dependency on traitlet >= 5 means that actually, the comm project requires python >= 3.7 contrary to what is published (claims to support python 3.6)

I've seen that the traitlet dependency is used just in order to import this : https://github.com/ipython/traitlets/blob/main/traitlets/utils/importstring.py , which is pretty stable and easy

I suggest to just copy the method in comm package and remove the dep on traitlet all-together
what do you think ?