Update flake8 requirement from ==3.8.* to ==4.0.*

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@3.8.0a1...4.0.1)

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

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

Author: dependabot[bot]

requirements-test.txt

@@ -2,6 +2,6 @@ pytest==6.2.*
 pytest-cov==2.11.*
 black==20.8b1
 isort==5.7.*
-flake8==3.8.*
+flake8==4.0.*
 mypy==0.790
 pytest-vcr==1.0.2