Update flake8 requirement from ==3.8.* to ==3.9.*

Updates the requirements on [flake8](https://gitlab.com/pycqa/flake8) to permit the latest version.
- [Release notes](https://gitlab.com/pycqa/flake8/tags)
- [Commits](https://gitlab.com/pycqa/flake8/compare/3.8.0a1...3.9.2)

---
updated-dependencies:
- dependency-name: flake8
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

requirements-test.txt

@@ -1,5 +1,5 @@
 mypy==0.812
-flake8==3.8.*
+flake8==3.9.*
 black==21.4b2
 
 codespell==1.17.1