chore(deps-dev): 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>

Author: dependabot-preview[bot]

poetry.lock

@@ -35,7 +35,7 @@ toml = { version = "^0.10.0", optional = true }
 pytest = "^5.4"
 hypothesis = "^5.1"
 isort = "^4.3"
-flake8 = "^3.7"
+flake8 = "^3.8"
 black = "^19.3b0"
 docformatter = "^1.3"
 mypy = "^0.770"