Update flake8 requirement from <6.0.0,>=3.7.7 to >=3.7.7,<7.0.0 (#825)

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@3.7.7...6.0.0)

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

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

requirements/dev.txt

@@ -1,6 +1,6 @@
 # development dependencies (see docs.txt for documentation building dependencies)
 # linting and static type-checking
-flake8>=3.7.7, <6.0.0
+flake8>=3.7.7, <7.0.0
 mypy~= 0.900
 types-requests>=2.25.0, <3.0.0
 # testing