Bump flake8 from 3.9.2 to 4.0.1 (#842)

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:development
  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>

Author: dependabot[bot]

requirements_dev.txt

@@ -3,7 +3,7 @@
 # pip install -r requirements_dev.txt
 #---------------------------------------------------------
 coveralls==3.2.0
-flake8==3.9.2
+flake8==4.0.1
 pillow==8.3.2
 pytest==6.2.5
 pytest-cache==1.0