Add dependency on black for tests
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Sorry, I have just seen that it was already in `QUALITY_REQUIRE`.

For some reason it did not get installed on my virtual environment...
2020-11-27T21:46:52Z
https://github.com/huggingface/datasets/pull/908#issuecomment-734996621
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2020-11-27T21:46:52Z
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Add package 'black' as an installation requirement for tests.