Bump flake8 from 3.9.0 to 3.9.1 in /requirements (#818)

Bumps [flake8](https://gitlab.com/pycqa/flake8) from 3.9.0 to 3.9.1.
- [Release notes](https://gitlab.com/pycqa/flake8/tags)
- [Commits](https://gitlab.com/pycqa/flake8/compare/3.9.0...3.9.1)

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

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

requirements/lint.txt

@@ -1,3 +1,3 @@
 black==20.8.b1
-flake8==3.9.0
+flake8==3.9.1
 isort==5.8.0