Bump flake8 from 3.8.4 to 3.9.0 in /requirements (#808)

Bumps [flake8](https://gitlab.com/pycqa/flake8) from 3.8.4 to 3.9.0.
- [Release notes](https://gitlab.com/pycqa/flake8/tags)
- [Commits](https://gitlab.com/pycqa/flake8/compare/3.8.4...3.9.0)

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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==3.8.4
+flake8==3.9.0
 isort==5.7.0