Bump flake8 from 3.8.4 to 3.9.0

Bumps [flake8](https://gitlab.com/pycqa/flake8) from 3.8.4 to 3.9.0.
- [Release notes](https://gitlab.com/pycqa/flake8/tags)
- [Commits](https://gitlab.com/pycqa/flake8/compare/3.8.4...3.9.0)

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

Author: dependabot[bot]

poetry.lock

@@ -34,7 +34,7 @@ aresponses = "^2.1.4"
 black = "^20.8b1"
 blacken-docs = "^1.10.0"
 coverage = {version = "^5.5", extras = ["toml"]}
-flake8 = "^3.8.4"
+flake8 = "^3.9.0"
 flake8-docstrings = "^1.6.0"
 isort = "^5.7.0"
 mypy = "^0.812"