Bump flake8 from 3.7.9 to 3.8.3

Bumps [flake8](https://gitlab.com/pycqa/flake8) from 3.7.9 to 3.8.3.
- [Release notes](https://gitlab.com/pycqa/flake8/tags)
- [Commits](https://gitlab.com/pycqa/flake8/compare/3.7.9...3.8.3)

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

requirements-dev.txt

@@ -1,6 +1,6 @@
 wheel==0.33.6
 pytest==5.3.2
 pytest-cov
-flake8==3.7.9
+flake8==3.8.3
 autopep8==1.4.4
 freezegun==0.3.12