Fix License metadata in PyPi package

Hello,

the license metadata field for traitlets contains the full license text instead of only the license specifier in the resulting METADATA file within the dist-info part of the traitlets package.
This then causes the field on pypi.org to also try to print the full license text.

I have no clue what causes this, though. The pyproject.toml in the repository looks just fine. I've seen other projects that have the same problem with similar config (specifying the license file in the license field and the short name in the classifiers - pipenv for example) , but I've also seen projects that do this without problems, so I'm at a loss as to what causes this.

Maybe it's got to do with some custom packaging script of yours that I couldn't find.
I hope one of the maintainers might immediately know what the problem is now that it's known. (I think you barely check the Metadata on the pypi page or your built packages after releasing an update)

首先检查 pyproject.toml 和所有 packaging 脚本，然后构建 traitlets distribution 并检查其 dist-info 目录下的 METADATA 文件。将生成的 license 字段与预期的短 license specifier 进行比较，并验证生成的 PyPI metadata 不再显示完整的 license 文本。

由索引模型根据 Issue 内容生成。