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 ?

首先定位 comm 包导入 traitlets.utils.importstring 的位置，并将其与 issue 中链接的 traitlets 的 importstring.py 实现进行比较。在保留该包所声明的 Python 3.6 支持的同时移除依赖，然后运行现有测试套件；issue 未指定具体文件或测试。

由索引模型根据 Issue 内容生成。