Fix bug in function validate_type for Python >= 3.9
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2022-08-09T10:40:22Z
https://github.com/huggingface/datasets/pull/4812#issuecomment-1209213545
https://api.github.com/repos/huggingface/datasets/issues/4812
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2022-08-12T13:41:23Z
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Fix `validate_type` function, so that it uses `get_origin` instead. This makes the function forward compatible.

This fixes #4811 because:
```python
In [4]: typing.Optional[str]
Out[4]: typing.Optional[str]

In [5]: get_origin(typing.Optional[str])
Out[5]: typing.Union
```

Fix #4811.