Bump flake8 from 3.8.4 to 3.9.0 (#38)

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)

Signed-off-by: dependabot[bot] <support@github.com>

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

requirements-test.txt

@@ -1,7 +1,7 @@
 bandit==1.7.0
 black==20.8b1
 bump2version==1.0.1
-flake8==3.8.4
+flake8==3.9.0
 isort==5.7.0
 mike==0.5.5
 mkdocs-material==7.0.6