Update flake8 requirement from <6.0.0 to <7.0.0 (#4110)

Updates the requirements on [flake8](https://github.com/pycqa/flake8) to permit the latest version.
- [Commits](https://github.com/pycqa/flake8/compare/0.1...6.0.0)

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

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requirements.txt

@@ -16,7 +16,7 @@ uWSGI>=2.0.19
 whitenoise>=5.2,<6.5
 
 # Linting
-flake8<6.0.0
+flake8<7.0.0
 flake8-debugger==4.1.2
 isort==5.10.1