Bump flake8 from 3.7.9 to 3.8.1 (#1722)

Bumps [flake8](https://gitlab.com/pycqa/flake8) from 3.7.9 to 3.8.1.
- [Release notes](https://gitlab.com/pycqa/flake8/tags)
- [Commits](https://gitlab.com/pycqa/flake8/compare/3.7.9...3.8.1)

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

Co-authored-by: dependabot-preview[bot] <27856297+dependabot-preview[bot]@users.noreply.github.com>

requirements_tests.txt

@@ -1,4 +1,4 @@
-flake8==3.7.9
+flake8==3.8.1
 pylint==2.5.2
 pytest==5.4.2
 pytest-timeout==1.3.4