fix(ci): check imports with the same isort as pre-commit

flowchart LR
  DEV["developer commit"] -- "pre-commit: isort 6.0.1<br/>keeps exploded imports (trailing comma)" --> CODE["src/"]
  CODE -- "CI validate-mr: isort 5.13.2 from requirements.txt<br/>wants them joined" --> RED["red on 9 files"]
  FIX["this MR: requirements.txt isort==6.0.1"] -.-> CI2["CI and pre-commit run the same isort"]

CI's validate-mr failed at isort on every MR into this branch, e.g. !201 (merged). The hook and CI ran different isort versions, and they disagree about imports that end in a trailing comma. Pinning 6.0.1, the version in .pre-commit-config.yaml, makes them agree. One file, kubernetes/client.py, had a misplaced requests import and gets a two-line move.

validate-mr step, run in python:3.12-slim	before	this MR	this MR + !201 (merged)
isort	10 files	clean	clean
black	clean	clean	clean
flake8	clean	clean	clean
pytest common edgecloud / network	3 / 3 passed	3 / 3 passed	3 / 3 passed
pytest contract/	51 passed	51 passed	60 passed

black has the same kind of drift (24.8.0 in requirements.txt, 25.1.0 in pre-commit) but passes today, so it stays out of this MR.

After this merges, rebase !201 (merged) so its pipeline runs on top of it.