CLN: trying isort-dev
- [ ] closes #xxxx
- [ ] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

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Trying ```isort``` 5.0.0 which is still under development.
-0.5 on the cython edits here.  I'm fine with the non-cython edits.

Detecting unused imports in cython would be nice. Closing this as this meant to be a pilot, to see if we will have problems with ```isort 5.0.0```, I will open a similar PR trying isort master, after they will do some changes.