Set up isort in pre-commit hook + add style check to ensure imports are sorted according to PEP8

[rdimaio]

Description

Follow-up item from the Rucio Retreat 2024.

At the moment, we're loosely enforcing the PEP8 recommendation for import sorting - unsorted imports are noticed manually by PR reviewers, and manual reordering is requested to match the standard.

isort is a Python utility that automatically sorts imports, and also provides a GH action to perform a style check for import sorts.

• add style check as GH action
• add to contributing guide
• add isort to pre-commit hook

isort GH action: https://pycqa.github.io/isort/docs/configuration/github_action.html (seems like it can also be done via pylint https://stackoverflow.com/questions/22722976/import-order-coding-standard)

Motivation

From PEP8:

Imports should be grouped in the following order:

    standard library imports
    related third party imports
    local application/library specific imports

You should put a blank line between each group of imports.

Change

No response

[rdimaio]

Closing in favour of #6538