Migrate post-generation hook to pathlib

[browniebroke]

Description

The post-generation hook makes heavy uses of path operations and Ruff suggests to migrate them to pathlib in #5613

These suggestions can't be fixed automatically by Ruff, so in that other PR, I asked Ruff to add a lot of noqa (with the --add-noa option) where there was issues. However, that makes the diff bigger that it should...

This PR intend to fix all the PTH issues from that script, and that script only.

Checklist:

• I've made sure that tests are updated accordingly (especially if adding or updating a template option)
• I've updated the documentation or confirm that my change doesn't require any updates

Rationale