py: no need to specify src for ruff ≥ 0.6

[DimitriPapadopoulos]

https://astral.sh/blog/ruff-v0.6.0#migrating-to-v06

By default, our isort rules now search inside src/ directories when determining the list of package names that are likely to represent first-party code. This was always configurable, but it was suprising for many users that Ruff didn't understand this common project structure "out of the box".