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-preview[bot] <support@dependabot.com>

setup.py

@@ -12,7 +12,7 @@
 
 EXTRAS_REQUIRE = {
     "tests": ["pytest"],
-    "lint": ["flake8==3.8.4", "flake8-bugbear==20.11.1", "pre-commit~=2.7"],
+    "lint": ["flake8==3.9.0", "flake8-bugbear==20.11.1", "pre-commit~=2.7"],
 }
 EXTRAS_REQUIRE["dev"] = EXTRAS_REQUIRE["tests"] + EXTRAS_REQUIRE["lint"] + ["tox"]