Fix metadata validation

[albertvillanova]

Since Python 3.8, the typing module:

• raises an AttributeError when trying to access __args__ on any type, e.g.: List.__args__
• provides the get_args function instead: get_args(List)

This PR implements a fix for Python >=3.8 whereas maintaining backward compatibility.

[HuggingFaceDocBuilderDev]

The documentation is not available anymore as the PR was closed or merged.

[lhoestq]

good catch thanks !