chore(deps-dev): update blackdoc requirement from 0.3.8 to 0.3.9

Updates the requirements on [blackdoc](https://github.com/keewis/blackdoc) to permit the latest version.
- [Release notes](https://github.com/keewis/blackdoc/releases)
- [Changelog](https://github.com/keewis/blackdoc/blob/main/doc/changelog.rst)
- [Commits](keewis/blackdoc@v0.3.8...v0.3.9)

---
updated-dependencies:
- dependency-name: blackdoc
  dependency-type: direct:development
...

Signed-off-by: dependabot[bot] <support@github.com>

pyproject.toml

@@ -72,7 +72,7 @@ sphinx-copybutton = "^0.5.2"
 pytest-cov = "^4.0.0"
 coverage = "^7.2.0"
 black = "23.11.0"
-blackdoc = "0.3.8"
+blackdoc = "0.3.9"
 mypy = "^1.2.0"
 ruff = "0.1.6"
 pre-commit = "^3.3.1"