Update flake8 requirement from <4.1,>=3.8 to >=3.8,<6.1 in /requirements

Updates the requirements on [flake8](https://github.com/pycqa/flake8) to permit the latest version.
- [Release notes](https://github.com/pycqa/flake8/releases)
- [Commits](PyCQA/flake8@3.8.0...6.0.0)

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

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

requirements/development.txt

@@ -2,7 +2,7 @@
 # -------------------------------------------
 
 black
-flake8>=3.8,<4.1
+flake8>=3.8,<6.1
 flake8-builtins>=1.5,<1.6
 flake8-eradicate>=1.0,<1.5
 flake8-isort>=4.0,<4.3