Update flake8 requirement from <5,>=3.8.4 to >=3.8.4,<8

Updates the requirements on [flake8](https://github.com/pycqa/flake8) to permit the latest version.
- [Commits](PyCQA/flake8@3.8.4...7.0.0)

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

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

requirements.txt

@@ -1 +1 @@
-flake8<5,>=3.8.4
+flake8>=3.8.4,<8