chore: modernize pre-commit hook configuration (#33)

## Summary

`.pre-commit-config.yaml` was broken on three fronts. This PR fixes all
of them.

### What was wrong

1. **`rev: main` / `rev: master`** — modern pre-commit rejects floating
refs and requires immutable tags (or a SHA) so hook versions are
reproducible across contributors. Every hook in the file had this.
2. **Dead repo URLs.**
- `gitlab.com/pycqa/flake8` was archived years ago. `flake8` now lives
at [`github.com/PyCQA/flake8`](https://github.com/PyCQA/flake8).
- `timothycrosley/isort` was renamed to
[`PyCQA/isort`](https://github.com/PyCQA/isort) and the old org redirect
can't be relied on for hooks.
3. **`psf/black`** still works, but the upstream-recommended hook source
is now
[`psf/black-pre-commit-mirror`](https://github.com/psf/black-pre-commit-mirror),
which ships without black's full dependency tree → significantly faster
hook installs.

### Pinned versions

| Hook | Old | New |
|---|---|---|
| `pre-commit/pre-commit-hooks` | `main` | `v6.0.0` |
| `PyCQA/isort` (was `timothycrosley/isort`) | `main` | `8.0.1` |
| `psf/black-pre-commit-mirror` (was `psf/black`) | `main` | `26.3.1` |
| `PyCQA/flake8` (was `gitlab.com/pycqa/flake8`) | `master` | `7.3.0` |

All four are the latest stable releases as of this PR.

## Notes

- Hooks run in pre-commit's own isolated venvs, so the linter Pythons
don't have to match the project's `python_requires=>=3.8` floor.
- The existing `[flake8]` config in `setup.cfg` (`max-line-length =
140`, exclusions) is unchanged and still applies.
- pre-commit.ci was discussed but intentionally left out of this PR to
keep scope narrow. Easy follow-up if we want it.

## Test plan

- [ ] CI `check` env still passes (it runs `flake8` and `isort
--check-only` directly via tox, not via pre-commit, so this PR shouldn't
affect it).
- [ ] `pre-commit run --all-files` works locally on a fresh checkout
(hooks install from the new revs).

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