Update flake8 requirement from ==3.7.* to >=3.7,<4.1 in /src

Updates the requirements on [flake8](https://github.com/pycqa/flake8) to permit the latest version.
- [Release notes](https://github.com/pycqa/flake8/releases)
- [Commits](https://github.com/pycqa/flake8/compare/3.7.0...4.0.1)

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

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

src/setup.py

@@ -236,7 +236,7 @@ setup(
             'coverage',
             'coveralls',
             'django-debug-toolbar==3.2.*',
-            'flake8==3.7.*',
+            'flake8>=3.7,<4.1',
             'freezegun',
             'isort',
             'pep8-naming',