Update flake8 requirement from <4.0.0,>=3.4.0 to >=3.4.0,<8.0.0

Updates the requirements on [flake8](https://github.com/pycqa/flake8) to permit the latest version.
- [Commits](PyCQA/flake8@3.4.0...7.0.0)

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

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

setup.py

@@ -23,7 +23,7 @@
     ],
     extras_require={
         "test": [
-            "flake8>=3.4.0,<4.0.0",
+            "flake8>=3.4.0,<8.0.0",
             "mypy>=0.521,<1.0",
             "pytest>=3.2.0,<4.0.0",
             "tox>=2.5.0,<3.0.0",