Update dependency flake8 to v4

This MR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
flake8 (changelog)	^3.9.2 -> ^4.0.0

---

Release Notes

pycqa/flake8

v4.0.1

Compare Source

v4.0.0

Compare Source

---

Configuration

📅 Schedule: At any time (no schedule defined).

🚦 Automerge: Disabled by config. Please merge this manually once you are satisfied.

♻ Rebasing: Whenever MR becomes conflicted, or you tick the rebase/retry checkbox.

🔕 Ignore: Close this MR and you won't be reminded about this update again.

---

• If you want to rebase/retry this MR, click this checkbox.

---

This MR has been generated by Renovate Bot.