build(deps-dev): bump ruff from 0.15.5 to 0.15.6

Bumps [ruff](https://github.com/astral-sh/ruff) from 0.15.5 to 0.15.6.
- [Release notes](https://github.com/astral-sh/ruff/releases)
- [Changelog](https://github.com/astral-sh/ruff/blob/main/CHANGELOG.md)
- [Commits](astral-sh/ruff@0.15.5...0.15.6)

---
updated-dependencies:
- dependency-name: ruff
  dependency-version: 0.15.6
  dependency-type: direct:development
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

pyproject.toml

@@ -37,7 +37,7 @@ Homepage = "https://github.com/testing-cabal/testtools"
 test = ["testscenarios", "testresources"]
 twisted = ["Twisted", "fixtures"]
 dev = [
-    "ruff==0.15.5",
+    "ruff==0.15.6",
     "mypy>=1.0.0",
     "typing-extensions;python_version<'3.11'",
 ]