Bump flake8 from 3.7.9 to 3.8.1

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>

requirements/flake8.txt

@@ -1,4 +1,4 @@
-flake8==3.7.9
+flake8==3.8.1
 flake8-import-order==0.18.1
 flake8-bugbear==20.1.4
 flake8-commas==2.0.0