revert unpin isort commit
This PR reverts the change of 
https://github.com/huggingface/transformers/commit/fbc5bf10cfe4d4ca81f8daacc148b0abd51dda5a

Using the unpinned version of `isort` makes black and `isort` disagree in some cases. 
In this PR the unpinned version of `isort` leads to a falied code quality test:
https://github.com/huggingface/transformers/pull/3411 


This syntax is not support by PyPi unfortunately, so we had to revert this.<|||||>> This syntax is not support by PyPi unfortunately, so we had to revert this.

I see...is there another way of  getting the pinned version? 
In general, do we need isort? Doesn't black also sort the import statements? <|||||>Other ways are:
- bug the isort maintainer to release a version containing this commit
- push a new forked package to PyPI, like `isort-pvp` or `isort-black-compat` or whatever.

Yes we need isort.<|||||>I have also observed this issue with #3402