Fix time type `_arrow_to_datasets_dtype` conversion
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2022-07-04T16:26:37Z
https://github.com/huggingface/datasets/pull/4628#issuecomment-1173986016
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2022-07-07T14:08:38Z
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Fix #4620

The issue stems from the fact that `pa.array([time_data]).type` returns `DataType(time64[unit])`, which doesn't expose the `unit` attribute, instead of `Time64Type(time64[unit])`. I believe this is a bug in PyArrow. Luckily, the both types have the same `str()`, so in this PR I call `pa.type_for_alias(str(type))` to convert them both to the `Time64Type(time64[unit])` format.

cc @severo 