Update darker requirement from ^1.7.2 to ^2.1.1

Updates the requirements on [darker](https://github.com/akaihola/darker) to permit the latest version.
- [Release notes](https://github.com/akaihola/darker/releases)
- [Changelog](https://github.com/akaihola/darker/blob/master/CHANGES.rst)
- [Commits](akaihola/darker@1.7.2...v2.1.1)

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

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

pyproject.toml

@@ -114,7 +114,7 @@ autopep8 = "*"
 bandit = "^1.7.4"
 build = "*"
 cpu-load-generator = "*"
-darker = "^1.7.2"
+darker = "^2.1.1"
 flake8 = ">5"
 flynt = ">1"
 icecream = "*"