CI: update to isort 5 (#35134)
- [x] closes #35134
- [x] 0 tests added / 0 passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

can you rebase IIUC isort 5 is sorting imports that we not sorted before (in .pyx and imports not at the top)

maybe possible to break some changes here off into smaller PRs before updating isort? > IIUC isort 5 is sorting imports that we not sorted before (in .pyx and imports not at the top)
> 
> maybe possible to break some changes here off into smaller PRs before updating isort?

I posted a tracking list on the original issue. And I'll break this PR into samller PRs. > I posted a tracking list on the original issue. And I'll break this PR into samller PRs.

Thanks. I think this makes sense. when we introduced isort, we did it on small batches of files. These can be quickly reviewed and merged.