Bump flake8 from 3.9.0 to 3.9.1 (#776)

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-preview[bot] <support@dependabot.com>

Co-authored-by: dependabot-preview[bot] <27856297+dependabot-preview[bot]@users.noreply.github.com>

requirements-dev.txt

@@ -1,6 +1,6 @@
 pytest==6.2.3
 pytest-cov==2.11.1
-flake8==3.9.0
+flake8==3.9.1
 black==20.8b1
 ray==1.2.0
 cloudpickle>=1.6.0,<2