Pin Ruff to 0.15.22 to fix the Lint Python job.

Ruff 0.16.0 (2026-07-23) enables 413 rules by default, up from 59. This
repo doesn't set `lint.select` in pyproject.toml, so CI's unpinned
`pip install ruff` silently picked up the wider default set and started
reporting 90 pre-existing findings across pedalboard/.

None of those findings relate to Python 3.15 support; they're all
pre-existing code that the older default rule set never flagged. Pinning
restores a reproducible lint and keeps this PR to one subject.

Adopting Ruff 0.16 is worth doing separately: 72 of the 90 findings are
auto-fixable via `ruff check --fix` (7 more with --unsafe-fixes). The
alternative is to pin the rule set instead of the tool, by setting
`[tool.ruff.lint] select = ["E4", "E7", "E9", "F"]` to make the old
implicit default explicit.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01ND3oQGgms3aXVsoBikUr93