Update flake8 requirement from <5.1,>=4 to >=4,<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@4.0.0...6.0.0)

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

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

Author: dependabot[bot]

requirements/development.txt

@@ -4,7 +4,7 @@
 # -----------------------
 black
 feedparser>=6.0,<6.1
-flake8>=4,<5.1
+flake8>=4,<6.1
 pre-commit>=2.10,<2.21
 pytest-cov==4.0.*
 validator-collection>=1.5,<1.6