Update flake8 requirement from <5 to <6

Updates the requirements on [flake8](https://github.com/pycqa/flake8) to permit the latest version.
- [Release notes](https://github.com/pycqa/flake8/releases)
- [Commits](https://github.com/pycqa/flake8/compare/0.1...5.0.4)

---
updated-dependencies:
- dependency-name: flake8
  dependency-type: direct:production
...

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

pyproject.toml

@@ -50,7 +50,7 @@ test = [
     "pympler",
 ]
 check = [
-    "flake8<5",
+    "flake8<6",
     "flake8-black",
     "flake8-colors",
     "flake8-isort>=4.2.0",