Bump flake8 from 3.9.2 to 4.0.1 in /.github/workflows

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>

.github/workflows/constraints.txt

@@ -1,5 +1,5 @@
 pip==21.3.1
 pre-commit==2.15.0
 black==21.9b0
-flake8==3.9.2
+flake8==4.0.1
 reorder-python-imports==2.6.0