Consolidate inline tox dev dependencies into dependency groups

Follow-up to #1873 / #1875.

The `lint`, `typing`, and `reuse` tox environments currently declare their dev dependencies inline in `tox.ini`. Those inline deps could instead be defined as PEP 735 `[dependency-groups]` in `pyproject.toml` and referenced from tox, giving a single source of truth for dev dependencies.

A concrete motivation: `flake8` is currently duplicated, declared both in the `style` dependency group (added in #1875) and in `[testenv:lint]`.

This is a consolidation/tidiness change, not a correctness fix, so it is proposed separately from #1875. The exact grouping is open for discussion (for example, whether `pre-commit` belongs in the `style` group or in a group of its own).

Caveat

tox's `dependency_groups` key requires tox >= 4.22. CI upgrades tox, but contributors with an older local tox would be affected. The `docs` environment (which uses `-rdocs/requirements.txt`) does not convert as cleanly and can be left as-is.