chore(lint): retire Travis/Hound/pylint configs, move lint into pyproject.toml

[loonghao]

What this changes

The repository root still carried 2019-era tooling. This removes four config
files whose behaviour is either dead or already covered elsewhere, and moves
lint configuration into pyproject.toml so there is one place to look.

File	Why it goes	Evidence
.travis.yml	Python 3.8/3.9/3.10 lint is covered by pythonpackage.yml; tests through Python 3.12 by import-test.yml	pythonpackage.yml, import-test.yml
.hound.yml	Its only job was to run flake8 against the config above, which ruff now covers. The Hound GitHub App itself is still installed and needs removing separately — see below	Hound status on this PR head: success, 2026-09-27T17:56:20Z
.pylintrc	pylint was never invoked by CI or pre-commit	no reference in .pre-commit-config.yaml or .github/workflows/
.flake8	superseded by [tool.ruff] in pyproject.toml	flake8 only gained pyproject.toml support in 5.x; the old file existed because of that

Travis coverage, itemised

The only step in .travis.yml beyond install was poetry run codecov. Nothing
generated a coverage report before it ran, so the upload was a no-op. There is
no codecov.yml in the root and no Codecov badge in README.md. Dropping it
loses nothing; the Python-version matrix it declared is a strict subset of what
the two Actions workflows already run.

Ruff port is behaviour-preserving

• line-length = 120, matching the old max-line-length
• same exclude list (docs, examples, test, build, dist, venv, old)
• E, W, F — what CI's flake8 photoshop --max-line-length 120 actually
  enforced
• Q with double quotes — the codebase already satisfies it

Measured on photoshop/: flake8 7.1.1 reports 0 findings and
ruff 0.16.9 reports 0 findings. CI verdicts are unchanged.

.github/workflows/pythonpackage.yml now installs a pinned ruff==0.16.9
instead of calling flake8.

Not ported — needs a decision

.flake8 also configured flake8-docstrings with the google convention.
Enabling the ruff equivalent (D, convention = "google") reports
426 findings on photoshop/. Neither that plugin nor flake8-quotes was
ever installed in CI, so those settings were inert — but silently dropping them
would be the wrong default. This PR keeps them out and flags them for a
separate decision.

Follow-ups, deliberately out of scope

• pylint is still listed in the dev dependency group but is now unreferenced.
  Removing it touches poetry.lock, which this PR does not touch.
• black is configured with target_version = ['py37'] while the package
  requires >=3.8. Left alone.
• AGENTS.md and llms.txt are handled separately.

Scope guard

No change to poetry.lock, packaging, or source code.

Action needed from the account owner (outside this PR)

The Hound GitHub App is still installed on this account. It posts a Hound
commit status on every pull request head — observed on this PR at 2026-09-27T17:56:20Z,
success, "Smells good to me. Woof!", 0 s duration — so it has not been
analysing anything for a long time.

Deleting .hound.yml removes its repository-level config, but the app will
keep posting a status that no longer corresponds to anything in the repository.
Uninstalling it is an account-level action (Settings -> Applications -> Hound ->
Revoke) and cannot be done from a pull request.