Bump flake8 from 3.9.2 to 4.0.1 in /tools

Bumps [flake8](https://github.com/pycqa/flake8) from 3.9.2 to 4.0.1.
- [Release notes](https://github.com/pycqa/flake8/releases)
- [Commits](https://github.com/pycqa/flake8/compare/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>
diff --git a/tools/requirements_flake8.txt b/tools/requirements_flake8.txt
index dda3ddc..400d992 100644
--- a/tools/requirements_flake8.txt
+++ b/tools/requirements_flake8.txt
@@ -1,4 +1,4 @@
-flake8==3.9.2
+flake8==4.0.1
 pycodestyle==2.8.0
 pyflakes==2.4.0
 pep8-naming==0.11.1