Bump flake8 from 3.8.4 to 3.9.0 (#710)

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-preview[bot] <[email protected]>

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

Author: dependabot-preview[bot]

requirements_dev.txt

@@ -3,7 +3,7 @@
 # pip install -r requirements_dev.txt
 #---------------------------------------------------------
 coveralls==3.0.1
-flake8==3.8.4
+flake8==3.9.0
 pillow==8.1.2
 pytest==6.2.2
 pytest-cache==1.0