Bump flake8 from 3.8.4 to 4.0.1

Bumps [flake8](https://github.com/pycqa/flake8) from 3.8.4 to 4.0.1.
- [Release notes](https://github.com/pycqa/flake8/releases)
- [Commits](PyCQA/flake8@3.8.4...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>

Author: dependabot[bot]

requirements-lint.txt

@@ -1,5 +1,5 @@
 black ==21.4b0
-flake8==3.8.4
+flake8==4.0.1
 flake8-bugbear==20.11.1
 flake8-docstrings==1.6.0
 flake8-import-order==0.18.1