📦 build: move dev extras to PEP 735 dependency groups

[gaborbernat]

The docs and testing extras serve only development, yet as [project.optional-dependencies] they ship in the published wheel metadata and let anyone pip install python-discovery[testing]. PEP 735 [dependency-groups] fits dev-only sets better, since a group stays local to the source tree and never publishes. 📦

This moves both extras into the existing [dependency-groups] table, with testing becoming test and docs keeping its name, and drops the now-empty optional-dependencies. The tox environments that pulled these extras (env_run_base, docs, and dev) now reference the groups through dependency_groups instead of extras.

The testing extra is gone; use the test group. No consumer-facing install extras remain, so python-discovery[docs] and python-discovery[testing] no longer resolve.