revert unpin isort commit

[patrickvonplaten]

This PR reverts the change of
fbc5bf1

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:
#3411

[julien-c]

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

[patrickvonplaten]

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?

[julien-c]

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.

[mgoldey]

I have also observed this issue with #3402