Update flake8 requirement from <4.1,>=3.9 to >=3.9,<6.1

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@3.9.0...6.0.0)

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

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

requirements.txt

@@ -1,7 +1,7 @@
 # Development
 django-debug-toolbar>=3.2,<3.6
 django-extensions>=3.1,<3.2
-flake8>=3.9,<4.1
+flake8>=3.9,<6.1
 isort>=5.9,<5.11
 
 # Testing