Adopt Ruff, target Python 3.13, add type hints

[jontyms]

Modernizes the project toward the Python style guide: Ruff, a current Python, and type hints.

Tooling — swap to Ruff

• Replace black + isort with Ruff for formatting and linting.
• Add [tool.ruff] config (rules E, F, I, UP, B, line-length 88, target-version = py313).
• Update .pre-commit-config.yaml to the ruff + ruff-format hooks.
• Replace the black.yml workflow with ruff.yml (installs uv, runs ruff check --fix + ruff format, auto-commits).
• Delete isort.cfg; drop black/isort/pylint/seed-isort-config dev deps.

Python 3.13

• Bump requires-python to >=3.13 (was >=3.9,<3.12) and add .python-version.
• The old <3.12 cap was hiding removed-stdlib usage, now fixed:
  • distutils.util.strtobool (removed in 3.12) → new githubapp/util.py drop-in.
  • collections.Iterable (removed in 3.10) → collections.abc.Iterable.
  • Drop the unused operator.truediv import.
• Re-locked uv.lock against 3.13.

Type hints

• Annotate every function/method signature across app.py and githubapp/*.py. Loosely-typed github3 client objects are Any; everything else uses concrete types.

Incidental

• Fixed pre-existing lint surfaced by Ruff: raise ... from, == True, %-format → f-string, a mutable default arg, unused imports, and long lines.

Verification

• ruff check . → All checks passed; ruff format --check → clean.
• All modules compile and githubapp imports cleanly with the Keycloak backend on 3.13.9.

Out of scope (still open from the style guide)

• Flask → FastAPI, and sync → async. Flagged for separate discussion.
• tox/tox-gh-actions dev deps are inert (no config, no tests) — left as-is; can remove if desired.

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