Update flake8 requirement from ==3.5.* to ==3.7.*

Updates the requirements on [flake8](https://gitlab.com/pycqa/flake8) to permit the latest version.
- [Release notes](https://gitlab.com/pycqa/flake8/tags)
- [Commits](https://gitlab.com/pycqa/flake8/compare/3.5.0...3.7.7)

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

requirements/test.txt

@@ -5,7 +5,7 @@ pytest-pythonpath==0.7.*
 factory-boy==2.11.*
 mixer==6.0.*
 isort==4.3.*
-flake8==3.5.*
+flake8==3.7.*
 pep8==1.7.*
 # for checking html
 beautifulsoup4==4.6.*