CI: pin isort version
- [X] xref #35134
- [X] 0 tests added / 0 passed
- [X] passes `black pandas`
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry. __this is a CI PR. I don't think we touch whatsnew for those.__

### Scope
<s>Remove the `--recursive` keyword from `code_checks.sh`, so that the newest version of isort can run without errors (it does recursive sorting by default).</s>
Pin the version instead, because version 5 has changed the way isort sorts imports. Needs more work to switch safely.
