Bump flake8 from 3.9.2 to 4.0.1 (#413)

Bumps [flake8](https://github.com/pycqa/flake8) from 3.9.2 to 4.0.1.
- [Release notes](https://github.com/pycqa/flake8/releases)
- [Commits](PyCQA/flake8@3.9.2...4.0.1)

---
updated-dependencies:
- dependency-name: flake8
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
Co-authored-by: Tom Christie <tom@tomchristie.com>

Author: dependabot[bot]

requirements.txt

@@ -26,7 +26,7 @@ anyio==3.3.2
 autoflake==1.4
 black==21.10b0
 coverage==5.5
-flake8==3.9.2
+flake8==4.0.1
 flake8-bugbear==21.9.2
 flake8-pie==0.6.1
 hypercorn==0.11.2; python_version >= '3.7'